Schools may require that newly created classes are hidden from the Global Address List (GAL) in Exchange Online. Classes may be hidden through PowerShell. Use the instructions below to hide Classes created with SDS from the GAL.

Classes are represented in Office 365 as Office 365 Groups. In Exchange Online, where the GAL is built, they are called Unified Groups. Use the Get/Set-UnifiedGroup cmdlet to manage these groups through PowerShell.

Hide a single class

Launch PowerShell as an Administrator and connect to Exchange Onlineas shown below.

$UserCredential = Get-Credential
$Session = New-PSSession -ConfigurationName Microsoft.Exchange -ConnectionUri https://outlook.office365.com/powershell-liveid/ -Credential $UserCredential -Authentication Basic -AllowRedirection
Import-PSSession $Session -DisableNameChecking

Once connected, run the command below against the Group you want to hide.

Set-UnifiedGroup -Identity <UnifiedGroupIdParameter> -HiddenFromAddressListsEnabled $true 

Hide all classes created by SDS

Launch PowerShell as an Administrator and connect to Exchange Online as shown below.

$UserCredential = Get-Credential
$Session = New-PSSession -ConfigurationName Microsoft.Exchange -ConnectionUri https://outlook.office365.com/powershell-liveid/ -Credential $UserCredential -Authentication Basic -AllowRedirection
Import-PSSession $Session -DisableNameChecking

Once connected, run the command below against all SDS-created groups.

 $Groups = Get-UnifiedGroup -ResultSize Unlimited | ? {$_.PrimarySmtpAddress -like "Section_*"}
Foreach ($Group in $Groups) {Set-UnifiedGroup -Identity $Group.Name -HiddenFromAddressListsEnabled $true}
}

https://docs.microsoft.com/en-us/schooldatasync/hide-office-365-groups-from-the-gal

If you want to export all your users email addresses to a CSV so you can see who has which alias you can do this with this Power Shell Command

Get-Mailbox -ResultSize Unlimited
|Select-Object DisplayName,ServerName,PrimarySmtpAddress, @{Name=“EmailAddresses”;Expression={$_.EmailAddresses
|Where-Object {$_.PrefixString -ceq “smtp”}
| ForEach-Object {$_.SmtpAddress}}}
| Export-CSV c:\smtp.csv -NoTypeInformation

I have wrapped it here so just paste it in to Power Shell to run and this will show the Primary Email address and then all the alias addresses.

PowerShell Get-Mailbox display SMTP addresses

http://cscmblog.blogspot.pt/2012/02/export-to-csv-all-your-exchange.html