polite/sensitive session. (JG)

What you are looking for we call 'a workshop'.

Most people interpret 'session' as come-all-ye.

There are often specialised sessions especially at the bigger festivals like Whitby, and may be described loosely as 'beginners', 'intermediate', 'experienced'. However they are not always advertised as this, but looking at who is down to run the session is often a good indicator. More open sessions in the pubs are usually free for all, so you take pot luck!