I would like to report security weaknesses to my school in UK. I had managed to find security weaknesses without any exploits or other software or hardware.

I had look at similar question however problem is that it is very likely to find out that it was me, even if I would use an anonymous email, like suggested in this question, as IT department know that I have lot of knowledge on computer programming, network, security, and it is (possibly) higher than anyone else's so I assume that I would be called straight away. Teachers also have knowledge that I found other security weakness which did not impacted school policy at all therefor I had no problem with this one. Also security weaknesses require physical access, so I couldn't lie that it was done remotely

Another mentioned answer in already mentioned question, said to just ignore it, however I had found out that one of their computers had been hacked by someone else, and to tell how I found this out I would have to mention security weaknesses, or suggest that I was trying to hack them.

If there is a teacher or counselor you can trust completely, that you know will keep your name secret even if the school administration starts making threats about firing people, I'd go to them first and talk to them in private. They don't need to understand computers or security (and you don't need to go into detail about the issue), they just need to be trustworthy and good at navigating the administration politics in the school: you need advice about the personalities of the people involved and how dangerous it would be for you to report the issue. If they're at all wary of reporting, then you should keep quiet.

If someone with enough power gets embarrassed, they might start looking for someone to fire or expel (or, in the worst case, to have arrested), to give the illusion that they are in control of the situation. If you're friendly with and trusted by the administration and IT department, and you know they've supported students in the past even when it made them look bad, it may be less risky to share the issue, but I'd still recommend going through a trusted intermediary.

If you can't talk to someone you trust to keep your name anonymous and you can't report the issue anonymously (and it sounds like you can't), it is probably best for you to keep quiet. And that means completely quiet: don't talk about what you found on forums, don't tell your friends what you found, and don't try it out again in a few weeks "to see if it's been fixed:" you don't want to show up in any logs as having anything to do with this, especially if it gets exploited by someone else. It sucks, but start by protecting yourself.
