Drug tests

We have random drug testing at school, and the past 2 times, i've been called. The first time, I told them I couldn't pee. I didn't have to. Who has to pee at 9 in the morning?
Anyways, I didn't pee, and then the second time, I had to pee, but I couldn't. I get really nervous when people hear me pee. and that lady has to stand right outside the stall. I can't pee like that. And so I have to go again when they come back to test us. I have no idea when it is, and I already know i'm not going to be able to pee.

Do you guys go through anything similar to this?
I hate it!
It's embarassing!
Any tips?