Michael has raised some good points, but I still feel the LGW connection is a reasonable option. The primary criteria I used in suggesting this was Michelle's intent to "make this as painless as possible for the students". Using flights to LGW and then direct to TLS seemed "painless" (no rail trips or airport changes required).
Regarding the possiblity of missing the connection to the EasyJet flights, the same problem would exist if arriving in Europe at Amsterdam or Munich and then using budget airlines from there to TLS. However, if connecting flights are booked directly to TLS with one airline, this would be easier.
I suspect it's not likely that a group of this size will be travelling only carry-on luggage, so the problem of dealing with checked luggage is going to exist in either case.
I wouldn't find it a big problem to go through Passport control twice, so that was less of a priority when I made my original post. Aside from taking a bit of extra time, it's not a big deal.
Another possibility would be to go via Air Canada from Toronto. There's a connection in Frankfurt, and then Lufthansa direct to TLS. Since this is a Star Alliance connection, checked luggage will only need to be retrieved on arrival in TLS (and only one trip through Passport control!).
Each of the options certainly need to be explored further, but I just wanted to offer alternatives to the suggestions that had been listed so far.
Cheers!