I suspect your teacher objects to the concept of "needing more time" for technical reasons.
Saying you need time, implies that you can be in possession of time as if it is a physical thing (e.g. "I need more beans"). But time is not physical, so can never truely be possessed.
For that reason, she considers it to be slang.
In my opinion, this reasoning is false, since slang is not defined as such.
Here is wikipedia's definition:
Slang is the use of informal words and expressions that are not
considered standard in the speaker's language or dialect but are
considered acceptable in certain social settings. Slang expressions
may act as euphemisms and may be used as a means of identifying with