Actually, it's not necessary, as you can hear a lot of people dropping the 'e' in this special case where you want to say at which educational level you are. Look at this example:
I'm in third class (of, for example, elementary school).
Man dar kelâse sevvome dabestân hastam.
مَن دَر کِلاسِ سِوُّمِ دَبِستان هَستَم.
Man kelas-sevvome dabestan hastam.
مَن کِلاسسِوُّمِ دَبِستان هَستَم. (no 'e' between 'kelas' and 'sevvom'; pay attention to removal of 'dar').
(It's not wrong to add 'e' here).