Monday: First bus from Mont-St-Michel to Pontorson leaves at 06:14 and last bus at 18:25. Tuesday-Friday: First bus leaves at 07:32 and last bus at 18:25. Saturday: First bus leaves at 08:47 and last bus at 18:50. Sunday: First bus leaves at 11:42 and last bus at 20:50.
Weekdays: First bus from Pontorson to Mont-St-Michel leaves at 06:54 and last bus at 19:35. Saturday: First bus leaves at 10:00 and last bus at 19:35. Sunday: First bus leaves at 11:27 and last bus at 21:43.
So Monday is the only day you could take a bus from Mont-St-Michel to Pontorson in time to catch the 07:57 train. On any other day you'll need to take a taxi.
See the timetables (les horaires) at www.mobi50.com.