Tresping? Please help me..?

We have this corridor inside our school. No student uses the rooms by the side of the walls of the corridor. In the past two years, there was a wooden barrier for students to know that it is a restricted area. But now the wooden wall was removed. I went to the corridor and was seen by a teacher. The administration tarnished my name by accusing me of tresping. But everyday, I see children playing by the said corridor but the only difference is that I was seen. I felt so cheated. Can you please show me a rule/law that I can use to defend myself. A rule/law that is applicable to any school. Something like, a place will only be recognized as restricted if there's a sign or whatsoever. Please help me.. I really need it.. Thank you in advanced.