I've used the MASC in the past. I find it to be more useful with younger children than with adolescents. I do also find it underestimates anxiety symptoms in children who are particularly not introspective, or who are reticent (e.g., those on the autistic spectrum, those who are more withdrawn, those with some dissociative/detached qualities).

Remember that a diagnosis should never be made based on a single data source.

...pronounced like the long vowel and first letter of the alphabet...