I think you're all really lucky to have lockers, security guards, and such.
I think if nothing else, searches would be a good deterrent to stop most students placing dangerous articles in their lockers. As for teachers/security guards stealing from students this isn't really an issue of locker searches and is more an issue of staff corruption and could be possibly be prevented by adding additional security measures such as CCTV with 48 hour storage and rules that no valuables are stored in the lockers.
Regardless, my 4 years of high school before I bought a car were a nightmare carrying around about 6 text books on my back all day every day. I would've loved a locker even if there were constant searches and we were restricted to only storing those books.
IMO they are perfectly fine. Sadly, the richest schools around where I live have the most drug trafficking, and there's no easy way to curb that; searching lockers is not illegal nor morally incorrect if its in the name of helping the learning environment (or general safety) of the students in the school. The lockers are school property anyway, and them being a safe haven for drugs and contraband would be asking for disaster.
Considering it's the school's property and you're just "borrowing it" they find it acceptable to search their lockers. I think it's a good precaution. When I was in high school, we had a few people who had brought drugs and dangerous weapons to school. Luckily we had officers on campus as well as drug dogs to take care of the situation. Our school even resorted to having drug dogs walk through the parking lots and if a vehicle was suspicious, it would be searched. If you denied them the right to search your car, even more police would be involved.
I think they have every right to search lockers, these are their property and are loaned to students so they can store school supplies.
Students should not store things there that are not school material (unless it's necessary things like medicines and stuff) in my opinion.
Students should sign a contract or something.(In my High School you just have to pay for the locker)
They should have conflicting students blacklists or something like that.