Praxis Grammatica - Playing Hooky

The 17th century Praxis Grammatica tells you how to tell your classmate to get off his high horse:

[ 18 ] Hodie non fui in schola, nec tu fuisti in ea. [English Translation]

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Praxis Grammatica gives great examples of Latin sentences in the guise of wondrously useful advice. Please feel free to post your interpretation of this bit of Latin and when it would be used.