Schools are often designed to BS9999 because, amongst other less onerous recommendations, this route is less likely to lead to a recommendation for sprinklers and compartment sizes can be as large as 8,000 sq m instead of BB100's 800 sq m.
And yet, BB100 is an entire book aimed at providing guidance for school design and BS9999 makes only a handful of references to schools and does not address them directly at all.
The introduction in BB100 makes it quite clear that it is the guide that should be used for all school design. BS9999 makes no such claim. As I said, BS9999 does make a few sideways references to schools and, personally, I think that these should be removed. Some less scrupulous fire engineers take the fact that schools are mentioned in BS9999 to indicate that it is an appropriate document for the design of schools. BS9999 mentions the word 'chair' many times but I wouldn't use it to design a chair. It mentions the word 'snake' (once) but I wouldn't use it to determine if a snake I stumbled upon was poisonous or not.
I concede that schools don't get built nowadays without at least one atrium and they might contain theatre type seating so there are often parts of the school where the best guidance is from BS9999. But not the whole school.
The original post was about fire risk assessments and I appreciate that BB100 is not a FRA guide and that BS9999 can offer useful information regarding matters that fall under the FRA (for example, fire safety management policies) so I'm not saying don't use BS9999 for a FRA but I'm also saying that the base design, if the building is newish, should be expected to conform to BB100. [Again, I appreciate that a FRA is not likely to make recommendations for reducing compartment sizes or fitting sprinklers because the RR(FS)O is not set up to properly address these issues - but this is all the more reason for the AHJs overseeing new builds to adhere to the correct guidance].
One other thing, in the table of 'typical risk profiles' in BS9999 it has classroom down as A2 and I have seen fire engineers take that to mean that the whole school is A2. This is, of course, absolute nonsense for modern schools which hold open days, parents' evenings, adult learning events and other events where members of the public use the premises. Yes, a classroom might have the occupancy class A - how can you not be familiar with the layout of a classroom in which you are sitting?? But it would be an unusual school if all the different user groups where familiar with the layout.
And as for cloakrooms, I do have to agree with BB100 in considering that they should be separated from escape routes.
https://www.tes.com/article.aspx?storycode=363613