too long

Like i said in the title i want to keep a list hidden until another another list above has been selected, for example show student list after a class had been selected (so only student who belong to the selected class are shown)

<select name="class">
   <option value="">class1</option>
   <option value="">class2</option>
</select>

<select name="student">
  <option value="">student1</option>
  <option value="">student2</option>
</select>

Initially apply a hidden class (with display: none styling) to the second select list and then on the change of value (indicating a choice has been made) in the first select - remove the hidden class.

EDIT - as as suggested by @HerrSerker - the event listener is now in the JS as opposed to inline in the HTML.

Note that each select list has a selected disabled option - to allow for a blank option rather than the first option selected by default.

// add event listener for change of class select list
document.querySelector('select[name="class"]')
  .addEventListener('change', updateStudent)


//function to remove the hidden class and styling of the second select list
function updateStudent() {
 var el = document.querySelector('select[name="student"].hidden');
 el.classList.remove('hidden')

 }

select[name="student"].hidden { 
 display:none
}

<select name="class">
  <option disabled selected></option>
   <option value="">class1</option>
   <option value="">class2</option>
</select>

<select name="student" class="hidden">
  <option disabled selected></option>
  <option value="">student1</option>
  <option value="">student2</option>
</select>

</div>