I can't believe what I stumbled across...

Scrolling through my perks I noticed that I had neglected my Illusion skill. At the time I was at level 21. So, I decided to arm my left hand with the Muffle spell and my right hand with the Oakflesh spell (so I could also level up my Alteration). I started casting the Muffle spell. After casting only ONCE I leveled up to 22. What? One cast!? Then I cast it ONCE again and leveled up to 23! I don't know how many times I was able to cast only ONCE to level up my Illusion! Then maybe two casts to level up; then three, etc. I don't have the exact math but in a matter of 20 minutes I am now level 57 in Illusion. And, I just counted casts between levels 56 and 57--only 7 casts of the Muffle spell to level up!

Try it! Of course this enabled me to also level up my character rather easily also.

I have never cast any Illusion spell except using Muffle occasionally while wandering around, and my Illusion is level 86. The spell gives way too much experience.

yeah, Muffle is [censored] in how easy it makes illusion to level. The whole school becomes almost useless after level 40 anyway though aside from invis anyway though