Counter argument would be the opposite so it depends what the argument is… if the argument is “no, cellphones shouldn’t be banned within school premises” then the counter argument would be “yes, cellphones should be banned within school premises” then you would give some reasons to back up argument such as “yes they should be banned because they can be distracting, they can be used to cheat on tests, etc” and further explain your reasons. Or “no they shouldn’t be banned because “they can be useful in case of emergencies, they can be used for educational purposes, etc.”