I agree with cgardener. If you want to ensure your phone is completely yours, I suggest paying for the bill in FULL. Then you can at least pull the card of "Well I pay the bill and I bought the phone."
Of course they can still pull the "well it's on our plan" card. In which case, there's two things I'd recommend.
1: set up to protect "thieves" from getting into your phone. If they try to get in and whatnot and it wipes itself, you can just say "well I didn't want thieves getting any of of my data; did you know people can use your phone to find out where you live and stuff? It's scary and I don't want that happening to me." type of deal
2: Opting for a prepaid android. Sure you won't have the best of the best android, or even the best signal(probably) but you will have the bill in your name and it will be all your own, no strings attached(and a cheaper monthly bill). Then if they want to get into your phone, you can just note that the phone is in your name AND the bill is in your name.
Even so, both things have flaws, they can ground you(although with option 2, it's less likely that they can/will ground you from your PHONE, because it's all in your name). If they take away your phone provided you own it and such, it's "technically" stolen. You can note that to them if they try that(although if you did that, I don't think it would make your life an easy one xD).
Also, you could just make yourself look like the most boring person EVER(I employed this technique and my dad asked to see my dumb-phone much less often leading up till the time I was 18).
There ARE ways to get a new default launcher to hide any social/journal apps. Quite a few of them free. I do like GO Launcher, it's pretty good at this.
Disconnect facebook, twitter, well any social networks notifications. If they go in your phone and see you have updates, then they will see you have those apps installed. Manual refresh sucks turds, but hey, it saves battery life!
Connect your school email(or personal email) to the phone and use it for mostly educational stuff(and scholarship applications, your job and such). If you have a more juicy email account, for crying out loud, use the web browser(and tell it to not remember your login info xD).
Seldom send texts- tell all your friends to not text anything personal, but to call you if they need to tell you something even slightly inappropriate. Invest in a headset with a mic.
Perhaps DL the mint app for finance tracking and such(make you look financially savvy if you have a bank account, if not go for a ledger app).
I know it sounds stupidly painful to use such a powerhouse of an android in this way(as in not to it's full potential), but if you tell them that they are more than welcome to go through your phone and you look like you're all business and no play(well put a game or two on there) then they may get bored or become proud of you and start to respect your privacy(you might still have to deal with a random check every now and then tho).
Also, if you tell them they can look through your phone, then it puts the whole thing on YOUR terms. It shows you have nothing to hide if they want to snoop, or that you're too smart to let any grizzly details of your life get into their eyes and/or you really have nothing to hide.
Even so, I would advise against posting or sending anything that can get you in trouble.