This happened some years ago during the very last class called Laboratory Exercises in the freshman year of high school. The whole school year we were divided to the two-group teams and watched a lot of different things under the microscopes. The equipment worked pretty well and that was why for quite a while me and my team-mate had an idea of examining our own semen there one day. There were certain difficulties, we could not leave the classroom during the class, because the teacher was always afraid of us misusing some chemicals outside and while we (nobody knows why) thought the ejaculation must be absolutely fresh we could only wait for the chance teacher would leave for a while. We were seriously thinking about masturbating right in the room, and once I tried it without him watching but stopped, because even we were in the room corner it was incredibly risky, anyone could just come and see. On the other side we never did it together before.
But on the very last session the teacher was asked to go out, he left warning us to be quiet what partially worked, so the two of us quickly run out of the classroom with the glass things in our hands to the closest restroom. We have never been in that particular one and for our surprise it was just one toilet room that could be locked. There was no time to run to another wing of the building, we just looked at each other and with some kind of curiosity we unzipped our pants, pulled out our dicks and started to jerk off like crazy. In less than a minute we ejaculated onto a glass, it was interesting to see somebody else doing the same thing I did daily. Just a second after we got back to the classroom, still with the bulge in our pants the teacher appeared in the door. So we finally succeeded and we saw what we wanted.