First painful knees...now this.
CSCMC1 suggested getting a sniffer dog. The problem with that is everything's documented, including the request if done via phone. If anything is found in the house, who owns the house? Unfortunately that opens yourself up for too much including possible charges against the owner of the house, no matter how ludicrous and beside the point.
If you haven't already dumped the room over, I say give him a chance to get it all out of the house.
He's been though enough today.
As a first go around for consequences, ask him to suggest what he thinks should be done with him if he were in your shoes, mentioning all legal consequences that you are now opened up to because of his lack of good judgement.

Aside,
Having that stuff in school is just plain dumb in this day and age of drug-free school zones. This is a good example of how pot can alter good judgement calls. If I was ever to do any of that back in my school days, it definitely would have been in the privacy of someplace other than the school.

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85' 300D No inspection, No registration fees, Cheap insurance
"If my calculations are correct, when this baby hits 88 miles per hour, you're going to see some serious %$&^."