What book is about a boy still at school but recruited into MI6?

You're likely thinking of "The Recruit" by Robert Muchamore, the first book in the "CHERUB" series.

Here's what makes it fit your description:

* Boy still at school: The protagonist, James Adams, is 11 years old and attends an ordinary school.

* Recruited into MI6: James is secretly recruited into a top-secret organization called CHERUB, which trains young children to be spies.

Let me know if you'd like to know more about the "CHERUB" series!