Should I know that "rzesz" is feminine? Or is it an exception?
Now you know. -cz,-sz, ś ć ending nouns can be feminine or masculine and only way is to learn by heart or check dictionary.
check this discussions for more info
There are also few feminine words ending in -ń, -ź, -dź, -ż, -w, -l, -c, -b, -p.
Reminds me of the małe rzeczy, in particular, Sylwia Grzeszczak - Małe Rzeczy :)
Same, learn this word beforhand because of her
Why is przedmiot marked as wrong
Provided that you matched it with a proper form of the adjective (mały), it obviously should be accepted.