I really like Cassia and, although it is different from the very mainstream names you have listed at your local school, it isn't that out there or hard to pronounce or wildly pretentious. I think it would fit in fine. I'm sure there will be at least a few unusual names in every class, even if they're only ones from other cultures.
Other names that give me the same vibe and could also (imo) fit in with/are currently popular names:
Cassidy, Cassie, Casey, Cara, Clara, Ciara, Carys, Aurora, Alyssa, Alicia/Alyssia, Alexia/Alexa, Alannah, Aria, Ayla, Saskia, Elisa, Elina, Freya, Gabriella, Imogen, India, Julia, Joanna, Kaia, Kayla, Leona, Lydia, Melissa, Matilda, Natasha, Nevaeh, Nia, Nova, Olivia, Phoebe, Pippa, Piper, Rhianna, Ria, Sienna, Sierra, Savannah, Sadie, Thea, Tia, Victoria, Zara
Other names that give me the same vibe but might be too 'out there' to fit in among her peers (not that I personally think it matters all that much):
Cassandra, Calliope, Calista, Camilla, Carmen, Araminta, Arianna, Alayna, Danica, Demelza, Electra, Evadne, Ember, Fenella, Fabia, Francesca, Greta, Giovanna, Hebe, Honor, Inez, Isadora, Iara, Iona, Jacinta, Jessamy, Jocasta, Jemima, Julianna, Juliette, Karensa, Katinka, Keziah, Livia, Liara, Luana, Larissa, Lorelai, Leonie, Lavinia, Miranda, Natalia, Nyah, Octavia, Paria, Pandora, Paloma, Ramona, Rowena, Romilly, Sylvia, Saffron, Sorrel, Sapphire, Samaris, Tamara, Talitha, Tabitha, Vanessa, Xanthe, Xenia, Zeryn, Zelda, Zillah