Guy beside me in class got a boner

Okay so a good friend I've known for like 8 years sits beside me in a class we have together and it's 3 people at one table but it's supposed to be 2 so it's quite tight and our legs would rub a lot and everytime they did he would stop and shuffle around and that made me suss lol and had me lookin for boner signs. I also have quite big boobs so I'm used to guys looking at them and I've caught him doing a few times too but in class he would be leaning over to read off my book and I would look at him and see him staring at them! Then the teacher told us to all stand up bc that's how she likes to quiz us and he was HELLA reluctant to stand up and took a while and then stood up but avoided facing me and had the weirdest pose and kept adjusting his pants and holding them strange and bending over a bit so I knew he had one! Then she told us to pair up and I asked him to b my partner but he was quick to say no with no eye contact and went with another friend ACROSS THE ROOM? I'm just so confused. How would he get a boner from those small things and why leave?