DD started High School this year and had my old iPhone 4, when I have an iPhone 5s (it changed this weekend when I upgraded to a 6s and she's now got my 5s)
I have been with Vodafone for donkeys years and what I did was get her a sim only deal for £15.99 a month that is linked to my account. With it she gets unlimited texts and calls, and 4Gb data. If she hits 3Gb data I get a warning text, another at 500Mb and it cuts off if it's used. So far the first month nothing and this month she hit 3Gb (a week without wifi and a YouTube obsession!). I pay £30/month for mine with 5Gb data.
A smart phone was the only choice for her really as we live some distance from the majority of her friends and they all keep in touch over instagram & Oovoo rather than text. Plus she has a very boring 40 min bus ride to school on a coach with free wifi, so she tells me most of them spend the time watching videos. Unfortunately one of her friends has an older 'brick' phone and has been the target of some nasty bullying/piss taking because of it. As wrong as it is, I don't want DD to go down that route.
She knows the 'dangers' of the internet and I am happy that she is being careful. I'll revisit if I ever need to (and she knows that). Her instagram is linked on my phone and I have put password restrictions on her phone for Age 18/explicit stuff. Plus, well, I trust her, so I am happy to let her have it.
Her phone cover is a great one - A bastex cover we got on Amazon. The phone survived a slide and crash off the car roof the first week she had it!