How do I use the time function in Views Math expression?
If you are using the Views PHP Field, http://drupal.org/project/views_php, you can enter in custom php field information. I would take the original date variable and subtract the date as of now to get the value you want:

  //Just creating a random freshman grad date

  $graduation_year = (date('Y-m-d', strtotime('+4 year')));

  $now = (date ('Y-m-d', (strtotime('now'))));

  $diff = abs($graduation_year - $now);


Based on the absolute difference in years you can do validation in your views field that validates this information and sets the student as a freshman, sophmore, junior or senior (assuming highschool :) ) based on the $diff value of 1,2,3,4. 

That validation would look like:

if ($diff == 1) {
print('Senior');
}
elseif($diff == 2){
print('Junior');
}
elseif($diff == 3){
print('Sophmore');
}
elseif($diff == 4){
print('Freshman');
}


I didn't put exception handling but you get the idea.