Either there is a spelling rule they're learning or there isn't.
As with many English spellings, there is and there isn't.
The u/oo spellings are tricky
a) partly because up North and including Liverpool words with , like
'bus, cup, shut', are pronounced with the same sound as 'push, pull, full'
or 'wood, wool, could' which is usually called short /oo/ (as opposed to the long /oo/ of 'moon, soon, spoon') in standard English.
b) because the short /oo/ sound of standard English has no spelling of its own.
All the spellings used of short are more common for other sounds:
'pull - dull, gull; should - shoulder; good - food, mood'.
Fortunately, only 36 words have a short /oo/ sound in standard English:
Good, hood, stood, wood.
Book, brook, cook, hook, look, rook, shook, took.
Wool. Whoosh. Foot.
Could, should, would.
Cuckoo.
Bull, full, pull, bullet, bullion.
Bush, cushion, push, shush.
Put, butcher, pudding, pussy, sugar.
Wolf, woman. Courier.