As said, most schools don't force you to shower. Hell, the only time I ever saw someone use them during gym was when we ran the pacer. The guys who did were sports buddies anyway so it wasn't anything they hadn't seen before.
If you are forced to shower try to go into that semi-erect position. You know, where it gets bigger but is still pretty floppy. Then of course you do need to have that ability. Some men don't expand when they power up. Sucks to be them!