The lesson types priority order in the Student Lesson table are;
E = Added using Edit/Add to list of student classes (Classes to Students display)
O = from Student Options,
S = White card (Cut and Paste) multi class groups, (Students to Classes display)
B = Band (Students to Banded Classes display)
C = Core class/Roll Class
A student could be assigned to an ‘S’ type class that is happening at the same time as an ‘C’ type class, the 'S' is the priority