Tell them that during the week you try hard to be responsible, get good grades, etc... but don't just say it, do it! Then tell them that it's important to you that after working so hard to be good and responsible, you get to look forward to doing something that you really love
.
Reassure them that it won't take you away from the things that are really important in life - like your responsibilities, and your friends and family. Then if they give you a chance, make sure that you keep your word so that they don't ever regret your playing Toontown.
Good luck!