Thank you for posting in the Community. The feature to mass email an entire grade level at once is currently not built in eSchoolPlus.
We also encourage the other Community members to share their insight if they have been able to find a solution to this. You can Vote the Enhancement Request ESP-I-242 on PowerSchool Ideas Portal so that the product team can take a look at this.
You can search for ideas using keywords or the idea number ESP-I-242 in the Search Ideas field at the top right corner of the Ideas Portal.
Remember to give Kudos to suggestions that help you, If another user helps solve your issue, please select Accept As Solution on their post so others can see the solution, too!