In reply to @wtgowers
@wtgowers I'm not sure what hosts offer it, but it's a built-in feature of the WordPress software. "WordPress multisite" is the thing to search for. You want that plus the ability to add plugins, for MathJax.
In reply to @christianp
@wtgowers What isn't always clear is that there's wordpress.com, the free service run by the company that makes WordPress the software, which is open source and can be installed and run by anyone. There are plenty of companies offering hosting for WordPress configured as you like