T-Mobile has an unlimited texting plan for $15/mo. There is no minutes/data included with that 15. If you want voice minutes, you can buy them by putting extra money in the account and it will get used at .10/minute. Our teen has this plan and it works great. I don’t think teens really call each other much anymore. It’s almost all texting.
For example, start by putting $20 in the account. This gives you a month of texting and 50 minutes of voice. At the end of the month, put $15 in the account and you’ll have another month of texting and any unused minutes carry over.
I would recommend not getting a data plan. Rarely does a teen really need data service. It can almost always wait until they get home. If you get data, your teen will be continually checking facebook/tumblr/instagram/whatever whenever you go anywhere.