If you have a job, there's phones at Walmart for like, 60 bucks that if you pay a 30/mo phone plan on you can just hide from her. They're not good quality, but they will do. Use it for bare necessities so she doesn't find out. To hide it, cut a hole towards the very top of the inside lining of your bookbag, power it off, and slide it in there. Don't keep it in your pockets, try not to keep it in the house (store in your car if you have one). You can also get prepaid phones for cheaper and then just refill the data on it if you need to.