Melissa - ademom is right, but in the meantime, you can probably relax a little. No, do not file a 990. At this point, the IRS doesn't even know your group exists. Technically, a group that handles $$ like most PTOs is either a registered non-profit, or a business. If you view your group as a business, you should be paying federal income tax. But like I said before, at this point, the IRS doesn't even know you exist. Now take some time to get educated, and consider your group's future.
Yes, with the kind of $$ you describe, your group should strongly consider incorporating and then filing for 501c3. But it's extremely unlikely the IRS is going to come calling on its own. The worst case scenario is when a group IS a 501c3 but they fail to file the annual 990/990EZ. THAT'S when you need to panic and act quickly to work with the IRS and hopefully get fines waived. But that's NOT your sitation. However, it does highlight one of the key aspects of getting 501c3 status--responsibility to the IRS. A group needs to have the organizational maturity to file the 990/990EZ EVERY YEAR once it earns 501c3. As Zaedeux is experiencing, the fines are very stiff for a lapse in filing.
That said, you should have a discussion with your fellow officers about formalizing your group. Is it the right thing for your group to do at this time? Once we learned that our PTO should be a 501c3 (due to the type of work we do, the $$ involved, the way we operate), we had a hard time ignoring the ethical obligation. But that's a decision for your group to make on its own.
There are tons of groups (not just PTOs) all other the place that are in the same situation as your group, mostly due to ignorance. The good news is that you have a little knowledge, a lot of resources, and the desire to do what's right.