I don't know about you, but when I text message, I do other things as well. I can easily send that many texts in a month without texting during school. If you are having a conversation with someone, you'll be sending way more than 8 texts an hour. And it's not the only thing you do. I do my homework and text at the same time, and get straight As. If the text messaging begins to become a problem, such as her grades dropping , or shes texting at a time that you, as her parent, find inappropriate (that is for you to decide, not for this forum to vote on) than the texting may be affecting her life, and then I could see taking her phone away as an option. But if she does what she's supposed to, and gets good grades, then you shouldn't punish her for talking to friends.
She lost her iPhone privs for a week over some issue; it's been a terrific motivator to ensure that she does her chores and has her homework done.