Two punctuation suggestions. 1) You should not have two colons in one sentence. So, you need either a period or a semi-colon after "serious issue" (or a comma and "because"). 2. When you follow a colon with a list, the elements of the list are separated by semi-colons, not commas (especially when each element is a clause, as in your example). So, it should be, "all these bad things can occur: they can fail their courses; their teachers can no longer trust them; . . ."