You must be 18 or older to receive a tattoo UNLESS your parent or guardian wants you to get a tattoo to cover up an illegal tattoo that is offensive.
In other words, if you have no tattoos at this moment, and you're under the age of 18, you're not going to get a tattoo from a reputable, legal, tattoo parlor. They can lose their license for tattooing a minor.
Whether or not your school would allow it isn't really relevant because it's illegal to even get one.
This is for educational purposes only. This answer is given based on the facts provided above. Contact a local attorney to further discuss these issues. No attorney client relationship is established. These questions and answers are fully searchable and viewable on the internet.