This is a tricky question — from a lot of angles.   The simple answer is that a person who is here in the US in B status cannot attend school.  And, if you tell the Immigration Officer at the Border that you are planning on attending school, he will not let you in.

But, as always, it is not that simple.   You definitely cannot attend a regular school in its regular program.   In order to attend school, you need to switch to a F visa, and you are absolutely not allowed to enroll before your change to F status is approved.   But, what if you just visit the class and attend without being enrolled?   If you are not registered as a student, you are not “studying”, you are just enjoying learning for the sake of learning.   But, if you do this while you are waiting for the change to F, and then once the change to F is approved, you try to get credit for your attendance at the class before the change to F  — that gets sticky.  You probably shouldn’t do that.

By the way, you are not supposed to enter the US as a B visitor with the intent to change to an F (student) visa after you enter the US.  It seems that it is easier to enter as a B than to enter as an F.   So people do that, and it is frowned upon — but people do it anyway.

I know that the USCIS/DOS (Department of State) will let people come to the US on a B visa to attend programs that are educational but are also fun — perhaps a dance camp or a music camp or a science camp.   I think that, arguably, they can come for a m-ssions training program.   I can’t say that for sure it is allowable, but people do it — I don’t know what they say to the person at the border or at the Consulate when they apply for the visa.   Now, it gets stickier if that training program also qualifies to accept F visa students.   I think that if a border guard knew that someone was coming to the US to attend a training program at a school which also accepts F visa students, the Border guard would probably not let the person in, but I can’t say for sure.

There is a similar issue if the student coming for a m-ssions training program is coming on a B-1 m-ssionary visa.   Someone coming on a m-ssionary visa should be coming to be a m-ssionary, not to be a student.   I think attending a m-ssions training program could be a problem.   But on the other hand, you can make the argument that they training program also includes significant opportunities to perform outreach.

I hope that is helpful.  I really don’t have much guidance here because there is a clear rule, but there can be a lot of exceptions depending on how you shape those exceptions.   But, on the other hand, there is no guarantee that those “exceptions” will work and keep you out of trouble.

I hope this is interesting and helpful.   Remember that this is not legal advice.  It is just a summary of certain aspects of immigration law which may or may not apply to your situation.   I encourage you to consulate an attorney if you think any of this may apply to your situation.