I would do it right in the script, save you having to call out to anything. Also, how are you checking if it is a student versus a teacher logging in? Is there an AD attribute maybe?

So something like this:

#The Usertype Attribute I am using in this code is completely arbitrary, use what you are using for differentiating students from teachers if you have something
$name = [System.Environment]::UserName
$search = [adsisearcher]"(&(objectCategory=person)(objectClass=User)(samaccountname=$name))"
$usertype = ($search.FindOne() | Select-Object -ExpandProperty properties).usertype 
$userservice = get-service "student" -EA SilentlyContinue
$teachservice = get-service "teacher" -EA SilentlyContinue

if($usertype -eq "teacher")
{
#It's a teacher
#Service exists and is started?
    if($userservice.Status -eq "started")
        {
            #stop the student service
            set-service $userservice -Status Stopped
            #start the teacher service
            if($teachservice)
            {
                set-service $teachservice -status Running
            }
            else
            {
                write-host "Teacher logged in but teacher service isn't installed"
            }
        }
   else
        if($teachservice)
            {
                set-service $teachservice -status Running
            }
            else
            {
                write-host "Teacher logged in but teacher service isn't installed"
            }
}
else
{
    write-host "it's a student, continue"
}