Come on. For math/physics/chemistry/CS the exams are easy. Just written questions and answers. Grading is also simple, just check if the answer is correct. For high-school-level questions, all the answers can be trivially checked. We're not talking about PhD-level problems.

It's a bit more complicated for humanities, grading essays is more subjective. But we also have solutions for that: have 2-3 people independently grade each essay, and have a special group review all the cases where the graders disagree.