What does the cryptic crossword clue "10 in school instructed from the back (5)" mean?

The wordplay uses a charade (building blocks). CO (synonym="CO") + X (synonym="10") + ED (synonym="school") = COXED The answer is COXED.

10 in school instructed from the back (5)

Guardian Cryptic #29366 | 16 Down | 2024-04-25 flower