After 5 years at Catholic school they don't know the Hail Mary.

[LynetteScavo]

They probably don't say it at mass every week as they are colouring in at children's liturgy.

They could rattle off the Our Father, and the morning prayer, lunchtime prayer, and the and the going home time prayer.

Should I complain to the head first thing in the morning? Or maybe they are just thick?

I am very surprised and a bit shocked. I would inform the head asap. It is a very important and lovely prayer and distinctive to our Catholic tradition. They should know it off by heart form the end of Year 1 imo.

Actually, it's only 3 years for DC2 & 3, and 5 years (2 and high school) for DC 1.

Still not good, though.

Should I talk to the head, or is it my responsibility to teach them?

Well it is both. I say it with my children every day. We also say it in school every day. I would highlight it to the head in a pleasant but concerned manner.

I would certainly have a strong word with the mother and father of the child as soon as possible.