Mistake #1 - allowing them to seach the car without a search warrant - Just say NO
One problem.
When you purchase your parking decal for the privilege of parking on the school property, you consent to the search in advance. Read the paperwork and the school handbook closely.
There was an issue with this in the past over Concealed Handgun License holders driving onto school property. CHL holders are not allowed onto the school
premise while armed. The question was whether the premise stopped at the threshold of the building entry, or at the property line. For purposes of the CHL,
premise was deemed to be entering the facilities (building, spectator area, playing field, etc). Meaning the parking lot was not considered "on premise". This goes against EVERY OTHER definition of "on premise" that Texas has. Example: An intruder is "on premise" the instant they cross onto the property, not when they enter a structure or facility.
This now brings into question whether the knife was actually on school premises. If this were my child, and he was in his car with its school parking decal (temp. or perm.) he and I had already consented to the search by affixing the decal to his car. If he was in my truck and had no permit decal (temp. or perm.) then the school would have to obtain consent to search.