Id give her a pencil and paper test at home. Either find some free ones online or get some nice workbooks, or write your own problems. Then have her do it in a quiet place and secretly time how long it takes her. She could be on track but very slow.

Any time the school principal calls you to tell you they are worried about your child’s academic performance, I’d be taking it very seriously. Schools only call you if it’s serious concern. Personally I wouldn’t be taking a wait and see approach if the school is sounding an alarm especially because around third grade things change from learning to read to reading to learn.