I had 4 chromis too, and they didn't school either, so you're probably right about needing more.
And just a little word of advice from another uncle...it's nice to get your niece involved in your hobby, but unless she lives with you (or close by) and is willing to share in the responsibility and joy of taking care of them, don't let her make ALL the decisions as to what goes in the tank. (my nephew would have had me buy 200 Nemos, a swarm of sharks and the Loch Ness Monster) Then when he loses interest in it, guess who'd be stuck caring for all of them. If you're the hobbyist in the family, do what you enjoy. Or get your niece her own tank with a simple setup and see how she does.
In order of setup:
FW1-16g, 5 Zebra danios, 4 Pristella Tetras, 4 Black Skirt Tetras, 2 Gold Barbs, 1 Brush-nose Pleco
FW2-5g, 1 Baby Gold Marble Angel (raised from fry), 6 Zebra Danios
SW1-75g, (80# LR
), Scarlett hermits, Turbo snails, Percula Clownfish (Gizmo), Oscellaris Clownfish (Whizzer), Indian Yellowtail Angelfish (Mikado), Royal Gramma (Orpheus), Black Sailfin Blenny (Casper), Yellow Tang (Pele), Racoon Butterflyfish (Raptor), Lyretail Anthia (Juliette)
FW3-30g, 2 mated Gold Marble Angels (Oscar/Yenta)
, currently empty
FW4-30g, currently empty
FW5-20g, 2 Gold Angels (non-paired)
SW3-35g (REFUGIUM for SW1) - EVIL ChocChipStar, Nassarius and Turbo snails
SW4-55g, 4 BlueGreen Chromis, King's Crown snail.