Grown adults still write 'an university'. They were taught as children that 'an' goes in front of the vowels a, e, i, o and u, and that rule is almost right. 'Almost' is exactly where the trouble starts. The real rule is about sound, not spelling: use 'an' when the next word begins with a vowel sound, whatever letter happens to be printed first.
Teach It With the Ear, Not the Eye
You're in luck at Grades 1 to 3. Almost every word children meet at this level has sound and spelling lined up, and apple, egg and orange really do begin with vowel sounds, so you can teach the sound rule straight away and never run into an exception. Here's the drill. Say a word and have the children rest a hand on their throat. If the first sound comes out with the mouth wide open (ah, eh, ih, oh, uh), that's team an. If a consonant sound leads the way, that's team a.
Children who learn by sound from the very start don't stumble years later over an hour or a university. The rule in their heads was right all along. All that has changed is the size of their vocabulary.
The A Versus An Team Game
Put an 'a' sign on the left wall of the room and an 'an' sign on the right. Hold up picture cards one at a time. Children run to the correct side and shout the whole phrase as they go: 'an egg!' Pick a category they have already studied, fruit or animals, and you revise two things at once. Then get it down on paper with a worksheet that asks them to write a or an in front of each word, and let them swap papers and mark each other's against the answer key.