It sounds like you've got a pretty good pension plan through your school system. I can't help but wonder, though, what would happen to the money if the system were to take a big financial hit. I'm not predicting bad things, but the truth is that you just never know what might happen. In other words, there's no way I'd lean on this fund as my one and only source of retirement income. Never put all of your financial eggs in one basket!
I wouldn't go as far as to pour an additional 15 percent into a different retirement plan, but I'd definitely look at putting seven to 10 percent into a Roth IRA. I want you to have some money that's separate from the school system account just in case things go south with their retirement fund. If you're married, and your wife works and generates income, she needs to be putting 15 percent into a plan, also. You can never have too much money during retirement!