Yes! Students only need one account, but they can join multiple classes in your school.
On the page where students can sign up and join classes, they will be given the option to create a new account (if they are new to Flocabulary) or use an existing account (if they have already created one.) Read more about this process.
If students will be using Flocabulary in multiple classes at your school, we recommend coordinating amongst your teachers so that students only create accounts one time. E.g., Students could create accounts when they sign up for their ELA classes. All other teachers would instruct students to then use their existing login. This way you won't end up with duplicate student accounts.