、。
Don't be lazy — use the kanji you've been taught.
Literal
Laziness do-not-[negative-request-ないで], taught kanji [object-を] use-[command-なさい].
横着 ('cutting corners, being lazy about proper procedure') specifically describes the laziness of taking shortcuts rather than general idleness — here, avoiding kanji one has already learned. The pairing of ~ないで (negative request) with ~なさい (gentle command) is typical teacher/parent speech: first correcting the bad behavior, then directing the right one.