My stepdaughter is 16 so I know how you feel.  What we do is tell her she cannot delete txts from her phone.  We monitor it which is alot of work but at least we know when she is doing something wrong.  We have caught her a few times deleting and she has had her phone taken for it.  What I do is check the amount of messages she has for the day and compare to online and see if they match.  After checking the number of txts and the txts themselves we delete them so the next day starts fresh.  We also use the usage controls so she cant txt during school and at night when she is doing homework and sleeping.  Hope that helps.  It is alot of work to keep up with the teens these days!