<p>I will be taking the class at ABCD High School.</p>
<p>OR</p>
<p>I will be taking the class in ABCD High School.</p>
<p>I think it is in.</p>
<p>Either is okay, depending on context. If you’re talking about the general location, then use “at.” If you’re talking about the specific building, then use “in.”</p>
<p>I think “at” would be the normal usage if you are talking about a specific school:
“I will be taking the course at Hunt Memorial High School.”</p>
<p>But if you were talking about when you’d take it, you might say, “I’m not taking the course now, but I plan to take it in high school.”</p>
<p>SATwriter is correct about this: “Where is the math class? It’s in Building B.”</p>
<p>But: “Which high school is offerering the math class this summer?” “It’s being offered at Smith High School”</p>