Does anyone know how the real play 'What's the matter with Angry?' ended

I take it you are new to the film. The direct answer to your question, is no. Apparently according to many of the posts the screen play is not available and of course the actual play goers are few and far between.
Your observations however are right on point and the reason the film became and still is a cult classic. It just ended leaving everyone in the air which led to the Fan Fiction on the web site with many alternative continuing scenarios. Contact in private message and I can give you many resources to explore.

No is the answer to your question as it very rarely gets revived here and the play isn't available to buy.

Honestly, I don't see how the ending is at all confusing. When Steven asks that question, he's implicitly imploring John to come out of the closet and be with him openly. Yes, John knows some of the school and his parents have an inkling of what went on but his background, status at the school and future plans means that the day after the speech, it would be very unlikely that anybody would speak a word of their suspicions and the implications of Steven's speech.

I just finished reading the original play. Will have to read it again to get the real meaning that Patrick Wilde had in his words, but while there are the same basic themes in the movie as in the play it is very different. The one real thing that is different is that John and Steve were not shy and coy with each other, they were very overt in their desires for each other right from the first direct and private meeting. The very end of the play has Steve being defiant as a happy to be Gay person and John looking at him but walking away as a committed heterosexual in practice. I know that is too short, but John had already ridden through being discovered and got everyone to ignore it. The play was far more realistic about gay practices and what happens to them. It could not have been directly made into a movie except for the XXXX houses.

To finish my thought of the play, it is absolutely depressing, unlike the Get Real movie which was uplifting and gave many of us hope.

I have just reread the play. The last scene is a white spot light on Steve dancing defiantly with his arms in the air. (The staging of the entire play was minimum furnishings, no scene changes and lighting to highlight the character of the moment.)In the shadows is John approaching him and then he walks away. John has already openly said that he really liked everything about what he and Steve experienced but that was a life he just could not lead openly. Hutton by the way is somewhat of a narrator throughout the play talking to the audience and at the beginning and the end talking to Steve in an attempt to keep him from doing anything drastic like taking his Mum's sleeping pills. Steve's parents got him to see a therapist as well.(See reason later.) Again all of the classic lines that we all love so much in the movie were in the play, but it truly was a very different setting. No prize day, no school mag, no Steven writing a newspaper article, etc. Oh, Steven's Father was molested by an older altar boy whom he ran away from. The altar boy became a priest. So his father wanted Steve to lie to his mother when they were told that he was gay by the head master. Steven instead broke down and admitted that it was true. Even his mother would not accept that it wasn't a phase that could be overcome.They had already been told by the school headmaster that Steven was gay and might be in a depressed mood as I said earlier. Hutton had told him but later regretted it. So the ending was typical as far as I can see and focused on the problems of English law at the time.

I have just reread the play. The last scene is a white spot light on Steve dancing defiantly with his arms in the air. (The staging of the entire play was minimum furnishings, no scene changes and lighting to highlight the character of the moment.)In the shadows is John approaching him and then he walks away. John has already openly said that he really liked everything about what he and Steve experienced but that was a life he just could not lead openly. Hutton by the way is somewhat of a narrator throughout the play talking to the audience and at the beginning and the end talking to Steve in an attempt to keep him from doing anything drastic like taking his Mum's sleeping pills. Steve's parents got him to see a therapist as well.(See reason later.) Again all of the classic lines that we all love so much in the movie were in the play, but it truly was a very different setting. No prize day, no school mag, no Steven writing a newspaper article, etc. Oh, Steven's Father was molested by an older altar boy whom he ran away from. The altar boy became a priest. So his father wanted Steve to lie to his mother when they were told that he was gay by the head master. Steven instead broke down and admitted that it was true. Even his mother would not accept that it wasn't a phase that could be overcome.They had already been told by the school headmaster that Steven was gay and might be in a depressed mood as I said earlier. Hutton had told him but later regretted it. So the ending was typical as far as I can see and focused on the problems of English law at the time.

One last highlight from the play: Dave had hidden a bottle of scotch in the loo earlier on the day of the first dance ever held at the all boys catholic school. Steven spotted it before the dance, drained some of it out and pissed in it. That was the bottle that John brought to Steven's house later after John told his date (sorta) that he had a headache. Steven knew the bottle and forcibly took the bottle away from John saying he had had enough and he really needed a better "blend". I thought that that was a riot. Dave and Kevin had been drinking from it during the dance.

. . . and that is one of the elements that (as I have argued here) moves this away from the "realistic" category. Although as someone else said, "Why would we expect a same-sex romance to be portrayed any more "realistically" than most on-screen romances are (which is . . . not very). I've also said that I think I might be a little older (and more American) than most contributors here - having just seen the film - and I can tell you with a reasonable degree of certainty that in America there would have already been strong suspicions about John by the time we join the story (let's face it . . . this is every closeted school kid's WILDEST fantasy). As Wendy says (in one of the film's more "realistic" lines), "You'll have to be more discreet . . . people aren't stupid." And most high school kids, in particular, have no better way to spend their time than speculate and gossip about things like this.
All that said, I enjoyed the film for what it was, and everyone gets an A for effort . . .