You need to edit a file in the etc/apache2/sites-available, called default i believe, in there you should see a reference to var/www that is just the root folder of your virtual apache host, change that to whatever directory path your new site is located in.
for example the file will look like (but with a lot more in the centre)
DocumentRoot var/www
ServerName http://www.schoolname.bham.sch.uk
The document root (in bold here) is the one you want to edit with the directory you need it to refer to. /var/www/joomla
I have tested this on my machine at home and it works with no issues, you can also add subdomains ie vle.schoolname.bham.sch.uk etc if your DNS server will able to forward wildcards.
Below is the article I used to learn these parts of Apache2, its pretty simple when you get your head round it!
VirtualHost Examples - Apache HTTP Server