
Quoting
Mr. Knowitall
If the school has a policy that all cars parked in the school parking lot may be searched, then students who don't want to have their vehicles searched should take the bus, get dropped off, or park elsewhere.
Searches may be conducted with permission, and if the school won't allow anybody to use the school parking lot unless they grant permission to have their vehicles searched, that would be a choice a student would make when opting to use the lot. Additionally, as long as they have a compelling reason to do so, schools can perform random sweeps or searches of parking lots and lockers for security reasons, without individualized suspicion. These days, making sure that there are no weapons in students' vehicles is likely going to be deemed sufficiently compelling.