What is the nuance of ‘Slipping’ when you say ‘I started slipping my classes short writing assignments?’

Solution 1:

The metaphorical use of 'slip' is pretty common.

• You can 'slip someone a mickey' (surreptitiously give them a much more inebriating drink than ordered so that they lose control (for nefarious purposes) (you can 'slip someone' any sort of intoxicating substance/drug).

• You can 'slip someone' some money, that is, handed them a little cash with the connotation that you're not doing it openly (not necessarily nefariously, just not in front of others

• Something can 'slip by you', if you forgot something recently.

Solution 2:

The meaning of to slip in this example comes from the transitive verb definition #6 in Merriam-Webster:

to insert, place, or pass quietly or secretly

It means that the instructor was including these kinds of assignments as a subtle way to teach students how to write concisely.

Another, more direct, example would be to slip someone a twenty dollar bill as a bribe:

I immediately got a table at the restaurant because I slipped the greeter a twenty.