As a past pres who 'took control', let me offer my two cents (okay, maybe three and a half cents is more like it...lol).
First, you are going to make people angry. You may loose friends. For a while... Think of your group like a new captain thinks of a ship, because that is just about the story in a nutshell. When the new captain comes aboard, there is always change. Especially when things are out of control or just plain old unorganized. Tough Love is not called Tough Love for no reason. In my case, I made a lot of enemies because I shook things up from the very start. I put tougher restrictions on almost everything, especially the bank accounts. I began with a membership of about 8, increased it to an attending membership of about 40 while also including folks who couldn't attend meetings. Most of the eight left because they didn't feel they were getting enough say. In the past they 'felt' that they were more in charge because the pres before me made them feel that they were making all the calls, when in fact she and the veep made all the decisions and were able (she was an expert at this...) to 'steer' them to the decisions she and the veep had already made. I was not as smooth, but they were actually getting more say than they ever had before and just didn't realize it. I set up a monthly meeting with the board to go over what would be presented (weeding out a lot of the garbage that when researched was worthless info for our group or our area). Instead of presenting 25 items, I had the board narrow it down to 3 or 4 'like' items. I set up committees, which had never been done. I just wasn't able to 'candy coat' things, didn't allow for hidden agendas, just said 'no' to cliques, etc. Needless to say I was not very popular with a lot of the old timers (all 8 of them...lol). Advice? Don't try to change it all at once. Try to walk a fine line between candy coating things and making actual change, but be firm in asserting your authority. You were elected for a reason. Somebody wanted change or you wouldn't be in your position now.
That brings me to my second point. Even if there appears to be chaos in the ranks, don't think that you are alone. Believe me when I say that even though it doesn't always look like you have support, trust me, you do. They may not be standing up with you in the public forum, but the support is there.
Third... It may just be me, but sometimes you just have to put your foot down. Just like with children, sometimes you have to put your foot down. Hard. Choose your battles, but once you have chosen your stand, Just Do It. Whether it be over the checkbook, fundraising, events, or whatever, you are in charge. You were voted in to be the one in charge, so Be The One In Charge. Next year, no one is going to remember what the teacher who went above your head did, but they are going to remember what you did or did not do. If at the end of the year you find that you are unpopular but made good decisions for the group, so be it. I may not have been very good at pleasing all the parents, but I assure you that the children all remember me and the things that were made possible during my year as pres. The teachers remember my year now, two years later, as remarkable because it set our group up correctly for the future. People who didn't like me very much then, now come to me for advice.
So... Hang in there. Fix everything that you can, because this is your shot. Don't quit unless you are willing for things to stay the same. Stand your ground. It may be hard for you right now, but think about the future of the group, the future of the school, and mostly, the future of the children. Remember that not everyone likes the captain, but it is because of the captain that a ship sails smoothly out at sea.
Good Luck!