All you have to do is use a scale which is small enough that it can't measure the weight of the complete bag. I guess that still doesn't prevent someone from w

All you have to do is use a scale which is small enough that it can't measure the weight of the complete bag. I guess that still doesn't prevent someone from weighing all of the candies, one small batch at a time, and adding up the results. But hopefully in a classroom it would be obvious if someone was trying to cheat that way. I suspect that asking the students to work in pairs helps to discourage those kinds of shenanigans, too.