I feel your pain. Sorry I can't offer any advise. My son suffered something very similar in 4th grade, the principle talked to the other child and the parents. Mom comes in screaming "my child didn't do anything".
Unfortunately that is what happens in a lot of schools. Even though they have a no tolerance policy, many principles look the other way. I think you need to go above their heads.

They are aware of it, but looking the other way. often Principals, like many of us just want to punch the clock, pick up the check and go home. I recently offered to purchase small trash receptacles and a years worth of liners to install in the girls toilets so they will have a place to dispose of their sanitary products. Young women are shy about carrying it out to the large
can in the sink area and my daughter and her friends have requested this small convenience. Anyway the principal(female) said "great idea, never thought of it, but yes I will order some, thank you for your generous offer, but I think our budget can do at least that much. Well, two months later....no trash cans. Multiple fundraisers, whining teachers and they won't accept a GIFT that would enhance the student experience. Oh I've also volunteered to provide lunches for kids I witnessed going without because the parents couldn't pay and are too proud to apply for the free lunch program. Again, no thank you. So they get no $$$ when the fliers come home. I'm pulling my child out to homeschool again - we'll just have to find other ways for her to spend time with her friends.

Are there locks on the lockers? I'm sorry to hear that any situation like this is occurring in any school, but it reiterates the need for written documentation about important concerns that parents have. I was advised never to rely on spoken communication when it comes to school matters. There are too many excuses for "he said -she said" (or in your case, claiming nobody said anything before!)

If I were you, even though several days have passed, I would write a factual letter along the lines of
"Dear Principal,
My son, (name), a student in (teacher's X grade class) has been a victim of theft and vandalism on 3 different occasions this school year. (List details, like his gym shoes were removed from his locker during 4th period P.E. on whatever date, and found in a urinal on date; etc.) I notified the teacher (name) after each occasion, and was told that security was investigating. I know my son is not the only student who has been the victim of this type of crime in your school this year. When I spoke to you (in your office on date) you said this was all news to you. If school staff is not communicating with one another, it is difficult to understand how the problem will be resolved....What is being done to improve the safety of my child and his belongings? Sincerely, Mr. and Mrs. Name."

I would send your letter via certified mail, with receipt requested, to the prinicpal, and copy the school district Superintendent. The principal can't "hide" from the issue when it's documented in writing, and his boss (the Superintendent) is aware of the situation. It would also help police to have written documentation if a similar situation should happen again.

Well Maybe contact the school board first and then the police I had experienced a similar situation and the school administrator did not want to be embarrassed so tried to keep the incident on the hush. Has an incident report been filled out? And had you been contacted when the incident occurred? I believe theft and vandalism is punishable by law and a direct violation of the student code of conduct. The administrator should be more proactive to resolve the problem. That is a shame and disgusting.