We used PhoneSheriff. There are many teen monitoring apps available. But I will tell you, once I installed it on his phone, I did tell him it was there but did not tell him all of the features. I immediately saw his friends texting him ways that he could remove it. He tried several. Their ways did not work. When I saw those text messages come through, I casually told him that if I saw that the app was removed, the phone would be gone permanently.
If access to the apps and internet is not a good idea right now, but she does need a phone to get a hold of you, maybe you could get her a flip phone or something similar that has zero ability to access the internet? My kids have had to tell their friends more than once they were grounded from their phones – and they’ve heard that from their friends more than once too.