declare @Classes table(Class int)
insert into @Classes(Class)
select 1 union all
select 2 union all
select 3

SELECT '<option value="' + 
	STRING_AGG([Class], '"></option><option value="') WITHIN GROUP (ORDER BY [Class] ASC) + '"></option>' AS Classes
FROM @Classes 

<option value="1"></option><option value="2"></option><option value="3"></option>

<option value="1"> 1 </option><option value="2"> 2 </option><option value="3"> 3 </option>