Was 'friends with benefits' through out my high school years with another female student, and I was over at her house and she was trying on clothes for a party we were going to that weekend, I'm just laying on her bed basically getting stripped teased, and then doing other activities. I ended up going home, but forgot to take my tie & mobile phone home, I get to school the next, and she brings my phone & tie to me. All good I thought, half way through the day I go check the time on my phone, and my wallpaper has been changed, to her in a thong with her arm covering her nipples while wearing my school tie - keep in mind this on an old Nokia which I think were like 2 or 3 megapixels at the time so not overly good quality but sexy none the less.
I told her she looked sexy in it, and then went home that afternoon just did my own thing, we had end of year exams the next day, I woke up late and entered the exam room basically as everyone was sitting down and forgot to turn my phone on off or on silent because I was more worried about the test not counting because I didn't show up on time. Any way half way through the exam my phone starts going off, examiner (English teacher) walks over to me and confiscates the phone. then I'm thinking to myself, no worries I'll just apologise for the disturbance at the end and say I was in a rush getting to the exam and forgot to turn my phone off and then I would get it back.
Completed the exam, and waited for everyone to leave, and went up to the teacher and asked her if I could get the phone back, she then looked mortified and said you'll get it back after I see the principal, I'm like da duck is going on, I thought someone might of SMS'ed me answers or something (even though it was the ring tone not SMS tone I was still stumped).
Next day I get called to the principals office with the teacher who took my phone, turns out I forgot to change the wallpaper and she noticed who she was, she kicked up a fuss, and eventually the assistant principal is like I'll deal with it, she leaves his office, and the principal is cool about it, his just like just delete the photo and we'll forget about it. I complied, and off I went. School returned a couple of weeks later, and another creepy teacher (who I didn't even have classes with) is like 'nice catch' or something along those lines, I'm like I got no clue what you're talking about, and his like Miss So'n'So told the rest of us about the photo(s), I'm like god damn, I didn't really get along with half the teachers any way so this will just make them even more annoying.
The girl in question did food tech/hospitality, and the teacher from that subject was like asking her all questions about it, and she got embarrassed because all the teachers basically knew that we were up to no good.
A couple of months later, I did the same thing at the start of a new school year started and I changed electives where it was basically a class full of chicks, and sometime in the lesson my phone goes off, and the teacher, with impeccable timing says 'any dirty pictures I should be aware of before I take your phone', class just bursts in laughter, and I laugh it off, and shes like 'don't worry, I won't take it, I'm scared of what I might find'.
Not overly embarrassing looking back on it but back then I had a few red face moments, as well as the girl I was semi-with.
td;dr: had sexy foto of another student, as fone wallpaper, teacher finds it, teacher tells all the other people, other teachers make jokes on my behalf.