Using the "ex" prefix on a multiple word subject

Solution 1:

Use an en dash to join terms that have spaces or hyphens. Examples from Wikipedia:

Pre–Civil War era
Pulitzer Prize–winning novel
The ex–prime minister

And yours:

He is an ex–school bus driver.
My ex–baseball coach taught me.

Alternatively, use former.

Solution 2:

According to the Chicago Manual of Style (here's a link to the CMS "crib sheet"), the following prefixes always require a hyphen:

• all-
• ever-
• ex-
• great-
• half-
• much-
• self-
• still-

Solution 3:

You could rephrase it: "he is a former school bus driver" or "he is a former driver of school buses" or "he used to drive a school bus."