The citric acid route does sound a good alternative if keeping concentrated acid solutions is not acceptable. Citric acid is found in fruit, and the kids might enjoy the fact that citric acid was produced on an industrial scale using mold shit. Instead of yeast beasties producing alcohol from sugar, we have/had molds produced citric acid from sugar.
Hopefully all chemicals, except those in use under supervision, are stored where they are not accessible by the students.
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