I reckon playing RuneScape can make your grades go down, but it won't get you stupid. I mean, how can your Intelligence ''go down''? The only thing I can say is:
-RuneScape = No real mental effort
-No Real Mental Effort = Lazy Brain
-Lazy Brain = Lower grades.
OR
-RuneScape = Playing Instead of Studying
-Playing Instead of Studying = You won't learn a lot
-You won't learn a lot = Lower Grades.
Personally i have always between 90 and 99% (100% doesn't exist in my school, no work can really be perfect) and I play RS a lot, but I don't think RuneScape > School and Social Life.
What I meant was your Intelligence can't really go down, but your prefontal cortex (nerdy, I know) gets lazier when you don't train it, but you won't permanently get dumb if you play a videogame too much.
Tricks for training your prefontal cortex:
1. Eat breakfast every day which contains carbohydrates, they give you some energy for the day.
2. Reading Aloud helps a lot.
3. Head Count is really good too.
4. Calculations and Syllabe count (doing it as fast as possible) helps too.
5. In conclusion, anything that doesn't let your brain become a lazy chewing gum is OK
P.S: Sorry if my syntax and some of my vocabulary is not good, english isn't my first language

but I'm still trying to do my best
Edit: I don't really think RuneScape is worse than any other addicting video game by the way.