I am starting mid-semester — what should I watch out for?

Two things: each student's remaining sessions, and the invoice issue date. A student with meetings left is given a credit pack the size of that balance, so attendance starts deducting from the right number. A student on a monthly subscription is simply placed into a class with a monthly pack.

For invoicing, set the school's issue date first, then decide whether the first invoice is prorated. The current month you already billed outside the system is simply skipped — automatic issuing only applies to the next period.