Do I have to get a data plan with an unlocked iPhone?

So I'm with AT&T right now but I didn't get an iPhone and have some stupid old free phone. I've been investing in getting an unlocked iPhone through Amazon and I was just wondering that if I do get the unlocked iPhone, do I have to get a data plan. Does AT&T need to know that I have an iPhone? I plan on using the Wi-Fi from my home as well as school to get internet so I really don't need to get Wi-Fi from them. Since I will get Wi-Fi from home and school, does that mean iMessage will still work via the Wi-Fi and not the data plan? It's just an extra burden on me if I need to get a data plan because I know my dad won't agree to it. Also, if I'm forced to get a data plan, what does this mean I have to pay for and what will be extra that I need to pay for? About how much more will I need to pay on my bill (our plan has my mom dad and I and my dad pays about 100 dollars each month.about how much higher would the bill go if i need to get a data plan)? So in conclusion, even though I will be using Wi-Fi from my home and school, and if I get an unlocked iPhone and still have AT&T, do I need to get a data plan? If I do, how can I avoid it if I can?

You SHOULD NOT get the unlocked iPhone. The unlocked iPhone is only good if you plan on traveling internationally (if you plan on having more than one carrier). And yes, you will need a data plan. The data plan is required for the iPhone, there's no way around it.AT&T doesn't know if you use Wi-Fi or not, so they assume that you'll need a data plan. The data plans cost…

300MB/$20 per month
3GB/$30 per month
5GB/$50 per month

I recommend the 3GB plan. So your total bill per month would be $120. P. S. Get the iPhone 5 in white!

Hope this helped!

You have to get data with any iPhone period.
Yes AT&T will know that you are using an iPhone on their network & apply the plan regardless.
Data plans don't give you Wi-Fi. They give you 3G/4G access.
iMessage works on either or (data/Wi-Fi)
You'd have to check AT&T to find out the minimal data plan
Can't avoid it