Well here goes nothing:
The other day I was in one of my classes learning about Chemistry. I had the urge to go before class started but I didn't want to miss my class. So class ends and I need to go but not to badly. There is a restroom outside, since the hallway is practically open and not closed off by doors.
I go through the double doors and stop in front of an open urinal. I'm all alone as I unzip and let out my penis and then someone walks in just as I'm about to start. He uses the urinal next to me. This causes me to tense up and only a few drops trickle out. A second person comes in and I tense up even more.
At this point I can no longer go even though I had to go pretty bad. First guy finishes up and second guy uses the urinal that the first guy just used. I was feeling awkward and somewhat embarrassed. So I pack up and zip up and leave. I couldn't go at all except for those few drops. I ended up having to go at my apartment.

Well that's my story. 

Why not use a stall? Unless your school dosen't have any?

Personally, I've never had an experience like this. The closest I can say is the few times I used a urinal with only one person in the bathroom, which makes me kinda nervous...

I can't piss at a urinal, I just can't go when others are around either.