Learn French: La soutenance de la thèse est stressante. - The defense of the thesis is stressful.

Stressante means stressful (causing stress), while stressée means stressed (experiencing stress). Since the thesis defense is what causes the stress, it is stressante. If you were talking about the student, you would say they are stressé or stressée.

What is the verb form of soutenance?

The verb is soutenir, which generally means to support or hold up. In an academic context, soutenir une thèse means to defend a thesis, making la soutenance the noun for the defense itself.

Does la thèse refer to any kind of school essay?

No, une thèse specifically refers to a doctoral thesis or dissertation, which is a massive research project. If you want to talk about a standard high school or university essay, you would use the word une dissertation.