If you school has an institutional account, joining is easy.
No ThingLink Account
If you don't have a ThingLink account, you will join the organization right at sign up. Go to www.thinglink.com/edu and register for a free teacher account. On the sign up page, make sure to enter your school invitation code. If you do not have this code, ask the administrator who setting up the school account.
Already on ThingLink
If you're already on ThingLink it is not too late. Head to your groups page (upper right menu > three heads icon). On the groups page, enter the school invitation code in the field on the right. Then click Join. After clicking join you should see a button to Request to join to the right of your existing groups. Click this button. Your administrator will need to approve your request.