I'd be more inclined to assume they're in the same year if she's casually saying that they could be in the same class next year. The less casual and more interesting bit is how she said it as if she could make it happen. She also apparently can leave guitars all over the school for her personal use, with permission. Maybe she has pull with the staff for some reason yet to be explained, and she's gonna try asking for a favor on class placement when they advance?