A nerdy unpopular uke, with glasses and black hair, handcuffs the seme (who is a hot, popular guy) to some pipes in the school bathroom and tries to seduce him. The seme is unimpressed and is certain he won't be able to "get it up", since he's straight, but doesn't mind letting the uke do whatever he want. At first the uke isn't successful in his tries, but after a while, the seme notices how inexperienced the uke is and he starts finding him cute/attractive and becomes aroused. So he decides to give it a try. At that point the uke becomes shy and kinda tries to get away, but they still end up doing it.

I think it might be the 2nd story in the manga, not the main one. That's probably why I can't find it.