I doubt the PCBs weigh too much. There's also a variance in battery weights. Testing it real quick my Li-ion AAAs weight 9g, NiCd 10g, and Alkaline 12g. There's also some Li-ions with built-in USB recharging circuits that I don't know how they effect the weight.

So better make sure they remove the batteries too. And who knows about the variances in internals they've done to the TI-84 in its 20 years of production.

Much easier - collect all student TI-84s, neatly labeled for return after the test. Hand out stock ones that already had a full reset completed. Don't bother to announce it until the student shows up for the test.

Students get the reassurance that their TI-84 won't go through a reset.

Schools have extra TI-84s in the back rooms.