There is no set schedule. It is pretty much guesswork going in, but you can attempt to make an educated guess about it beforehand if you follow the Twitter feed.
Haven't seen it in about a year, so I forget--what role is "tall boy"? I know who the small boy is, but can't think of the tall boy role and don't remember when a second Billy/Michael actor was in the show. Thanks.
Well when you saw it, "tall boy" was probably not played by a Billy or a Michael, that is a pretty recent thing. It was originally just played by Keean Johnson, who has now been bumped up to Michael, so instead of getting another kid to play "tall/posh boy" they have the Billy's and Michael's do it. The "tall boy" show up during the opening scene, running on screaming "He's heard!" then his dad comes and chases him off. Then he is the third boy in the beginning of the boxing scene (doing push-ups, it's Michael, tall boy, and small boy). Then he shows up at the top of the second act in Merry Christmas Maggie Thatcher, and as the "posh boy" he shows up at the Royal Ballet School when Billy punches him.
Tall Boy gets yelled at in the opening by the Scab Dad, and is the other kid in the boxing lesson who leaves after getting whapped in the face by George. He's also in Merry Christmas Maggie Thatcher, and most notably I guess in the audition scene as Posh Boy who gets punched by Billy.
And to answer the question about why they don't release a schedule, my best guess is so that people can't plan on seeing a certain Billy, that way there is no clear favoritism or anything I guess.