Your wording "people [...] being studied things" is non-idiomatic. If people are being studied then someone else is trying to learn something about them. From context, I think you mean people being taught things. That indicates that someone is trying to instruct the people in question ("taught" being the past tense of "to teach").
Merriam-Webster gives these four meanings for "class" in school context (along with several others in different contexts):
a : a body of students meeting regularly to study the same subject
Example: "My Biology class dissected frogs today." Perhaps this is what you meant by "People who are being studied things".
b : the period during which such a body meets
Example: "Turn off your cell phones during class." This seems to match your "period of time during which people who are being studied things sit and learn."
c : a course of instruction
Example: "I need only four more classes to graduate!" This is similar to a "subject", and in some circumstances and localities the two might be used interchangeably.
d : a body of students or alumni whose year of graduation is the same
Example: "This bench is a gift from the class of 2011." This one is not among the terms you asked about.
You also asked about "An ABCDEF", which would be called one of a "grade" or a "mark" in most English-speaking locales.
"A place where people study" could be a "classroom", but also many other things ("study hall", "library", "study", "bedroom", and many more).