Ah yes. Good tip from Ausette to indeed max only guitar first. But if you're like me and wanting your Sims to know absolute everything, you still will have problems with the other instruments. Let's say you've maxed both guitar as well as bass and drums for example, than piano would be even more difficult because you now have to deal with multiple opportunities that will come by eventually.
In my experience, try play the instrument you will fullfill the most at that time. Or even better, do nothing else than playing that instrument in places much people can see you, like at parties. I've seen that most opportunities that came when I was playing bass where bass related. Which seems obvious and beside that pretty usefull to know.
So only focus on one instrument at a time, and beside that, only let you Sims read books if you want to.