Be at school assembly or in school assembly

It depends on the context whether to use "a", "the", or the zero article, but you may say either "in or at (a, the, 'zero article') schoool assembly", "in" being more frequent (ngram). We can't make the distinction between "at" and "in" on a parallel with the distinction that is made in British English between "at school" and "in school" (ref.) because all students attend on a compulsory basis.

1/ If the associated educational context is such that school assemblies are regular happenings programmed by the school's staff to occur regularly, as is the case in the U.K., you are speaking of something of an "institution", an organisation that most people are aware of in that system. In that case, given a particular case of assembly, that is, given a particular gathering, if you still want to refer to the happening on the basis of its institutional foundation, the zero article is the normal choice. See "The zero article in American and British English" in this page.

• The students are at/in school assembly.

2/ If you are talking about a particular assembly of the school and want to single out this event as one among others, you use the indefinite article; you use "a" ("an") in this case because the specific identity is not known (see this page).

• The students are at/in a school assembly.

3/ If you are talking about a specific occurrence of gathering of the school assembly type, then, whether in an institutional context or not, if the particular assembly is one determined by the textual context, you use the definite article "the", also called the definite determiner "the".

Example

• The school assembly that had been postponed until the next day has finally been cancelled. (The context has it that a definite gathering has been cancelled, that given one that was first postponed to another date.)

In the case of the description mentioned in the question there seems to be hardly a case for the use of "the", unless the sentence is a transition from a text requiring or announcing the description of a picture to that picture, but then the sentence would probably be more aptly put if completed by an adjunct such as "that I am going to talk about".

• The students are at/in the school assembly that must be depicted.