Office365 Academic (Education) does not support organization-wide teams. To get over this limitation, you can maintain the membership by a simple PowerShell script:

Get-MsolUser -All | Where-Object { $_.Licenses.ServiceStatus.ServicePlan.ServiceName -match "TEAMS"} | ForEach-Object { [pscustomobject]@{User = $PSItem.UserPrincipalName} } | Add-TeamUser -GroupId f29dbfc0-d118-4480-a6f7-22048a0809d0 -Role Member

In short:

1. Take all Azure Active Directory users.
2. Filter them to those having a Teams license assigned.
3. Transform the data to input usable for Add-TeamUser cmdlet.
4. Add the users to the team (you will get the GroupId by using Get-Team cmdlet)

You can run the script repeatedly, just the missing users will be added. You can modify the script easily and apply any additional conditions when adding the users to the group.

Might help:

Install-Module MicrosoftTeams
Install-Module MSOnline

Connect-MicrosoftTeams
Connect-MsolService

Get-Team