There's the expression forum agitur - said of administrative sessions presided over by the governor of the province, so should work for official contexts. Very similar is conventus agitur - of big judicial sessions held by the visiting praetor. Otherwise you would specify the exact process/body/etc. that the session represents, such as causa, senātus... The former (a process) goes with agitur, the latter (a body) with habētur. Finally, the most all-encompassing word for 'session' is, well, sessiō "a sitting", but in the real sense of buttocks being applied to sitting surfaces. All the verbs here are impersonal "is being X".
edit: conventus in this sense is fine in the singular.