Try BS4163. There is also CLEAPPS guidance, and some stuff on the Department of Education website. (Or there was the last time I did something like this!) Obviously the establishment RA would be a main part, there would be no way to set any values without knowing the layout of the workshops etc. access, egress, space around machinery, how the workshop complied wrt PUWER98, & IMHO BS4163 also. An assessment of the workshops and machinery in the way of a PUWER98 assessment of the equipment etc. and then the additional requirements of BS4163 would give you a good idea on how fundamentally safe they are. From this the "higher the score" under PUWER98/4163 then the safer, thus potentially the more trainees could be accommodated per lecturer. Establishment ethics would also come into this, perhaps even student inductions, single point lessons, & mini "sub" training courses on specific machines in small groups, buy in from the students, etc. etc. all formally documented would help. If these are 16 yr old school leavers, in daytime college then potentially less than mature students on evening classes, (perhaps even younger students in evening classes). Evening class students are "generally" paying themselves to be there, so want to be there, so want to learn, so are generally less trouble, and tend to work "with" the establishment better.