。・・・。
Today's a school day. I'd better not be late...
Literal
Today [topic-は] school-day is-[copula-だ]. Late not-become-[in-order-to-ようにしない][must-と]...
A student's self-reminder. 登校日 ('school day' — literally 'day of going to school') is a specific term for days when students must attend, used especially during vacation periods when only some days are school days. ~ないようにする ('try not to X, make sure not to X') is a fixed pattern for preventive resolutions. ~ないと is the casual 'have to / must not' (contracting ~ないといけない).