I masturbated 3 times actually during school classes in my early teens.
I'm not sure whether anyone saw ... but no one ever said anything
I have masturbated in stalls in toilets in every place I've worked, in toilet stalls of train stations, pubs, clubs ... even strip clubs - before I realized that it was an option to pay a few extra pounds and get the stripper to do it for you! lol (I was so naive once!)
I've also done it in urinals, but only when either (a) too drunk to realize I could be caught any second or (b) in an empty building where I knew I wouldn't be caught.
I used to work in security and I'd masturbate a lot on night shifts in open spaces in the building I was guarding.
Over the year or so while I was doing the job, I masturbated in reception areas at the front of the buildings, always making sure I wasn't on security camera, naturally!
I also used to often find somewhere during patrols ... the buildings had first aid rooms with a bed ... I also made a point of trying to hit every toilet in the buildings at least once. Especially the ladies.
lol ... I feel I maybe sharing too much
Anyway, some of the buildings were quite tall and I used to go to the very top, in the stairwell and cum through the gap in the bannister so that my sperm would fly all the way down to ground level!
One building was over 30 storeys
and the sperm made it all the way to the bottom - I found the drops!
Lastly, I've been known to discretely stroke when in the back of a black cab - but never to orgasm ... yet.