For the first one, you'd have to change your preposition, but you could say breeze through.
Before I went to college, I could just breeze through school.
In addition to definitions about light winds, NOAD says:
noun informal a thing that is easy to do or accomplish : traveling through London was a breeze.
verb [ intrans. ] informal
come or go in a casual or lighthearted manner : I breezed in as if nothing were wrong.
• [ intrans. ] deal with something with apparently casual ease : the computer has the power to breeze through huge documents | he breezed to victory.
If you used breeze for Felix, though, one might interpret the sentence to mean that Felix tackles problems at work with relative ease (breezing through them), thereby giving the opposite impression than you intend. So, another suggestion might be:
I used to respect Felix, but now he just saunters around at work.
To saunter usually applies to a casual movement, rather than a casual attitude, but I think the word can be used figuratively to someone who is slacking. I'd probably be more direct, though, and just use slacking:
I used to respect Felix, but now he's always slacking at work.
Again, from NOAD:
• [ intrans. ] informal work slowly or lazily : she reprimanded her girls if they were slacking.
You could use slack in your first sentence, too:
Before I went to college, I could just slack in school.