Forbidden Time also goes by the title Kodomo no Jikan, a separate and more popular series which already has an article here. This was a series of 6 OVAs 30 minutes long released from 2000 to 2002, prior to the start of the separate KnJ manga in 2005. It also got a manga adaptation.
There is a bath scene with an older girl and 3 younger males.
In one of the OVAs, a brunette is in a hot spring full of men.