The general liability policies offered by the PTO Today (NPN) program and the national PTA have a liability limit of $1,000,000--I wouldn't suggest $100,000. Unfortunately, in today's society that figure is far too low. (General liability coverage is for when someone or someone's property is injured/damaged.)
As for officer's liability insurance (also called D&O coverage), you do still need it, regardless of your 501(c) status--that only refers to whether you pay income taxes, it has nothing to do with insurance. Again, a standard liability limit is $1,000,000; the deductibles on the NPN and PTA programs are $1,500.
You should also look at a fidelity bond (also called bond insurance). This protects the organization's money against embezzlement, theft, etc. Here, the limits are lower--you can usually purchase for as little as $10,000. How much you buy depends usually on what your budget is (i.e. how much money someone could actually make off with). The deductible on fidelity bonds for the NPN and PTA programs is usually $250.
Finally, you might need to look into property insurance if your group actually owns some equipment. Again, the liability limits there are much lower--like $10,000.
Just make sure when you are getting your various quotes that you have an apples to apples comparison in terms of liability limits, deductibles and exclusions. It's also a good idea to check the financial rating of the insurance company offering the coverage, which you can do at
www.ambest.com/ratings/
--you need to register, but it's free.
Good luck--the NPN program rates are not as inexpensive as the PTA rates, but probably better than anything you would find on your own.