I might be wrong, but I think having the child in the Very Focused mood also helps the speed of the mental skill increase.
I have an area of the household setup to radiate a very strong focused aura and always have the kids do homework and work on mental skills there....homework is done almost instantly, and mental has been very quick to raise compared to motor for the 4 kids raised there.
I set the area up mostly just using the lovely prints gotten from using the telescope, and a handful of the career rewards from tech guru. Remember you can shrink wall art down with [, so you can decorate a pretty small area with a lot of the prints that way. I have them all hanging in various sizes so that it's aesthetic as well as functionally providing me the very focused mood for any sims that go there.