I have a feeling the light spirit was taking up all her magic and couldn't keep 4 of such high caliber all at once.
Nah, all three did it for selfish reasons. The water spirit because FL spent time studying and didn't pay her enough attention. The fire spirit because he's just an asshole. The wind spirit was partially to alleviate the strain on FL, but like, 90% because...
Spoiler
He was originally contracted to FL's mother and has been looking for evidence for his revenge on FL's father, who arranged FL's mother's death by "carriage accident." He deemed it would be easier to find/record evidence if he switched to the sister. He also ends up playing audio (via his abilities) of the sister's true bitchiness in front of the entire school as a final gift of sorts for FL.
Spirits betrayed the FL, though it seems that two of them did it because FL didn't have enough magic power to give all three of them... Which was true only at the beginning a d thus became a bs real fast. Like, how did you not realise your master getting stronger? Did you just not give an f?
The art is mid, but the spirit magic seem interesting.