The grading of assignments is rather easy, once you find where to access the student submissions and have done it a time or two it becomes second nature. Here is a link to Schoology's support page on a few different ways to access student submissions and grading: How Do I Grade Assignments
The video at the right shows how to view and annotate user submitted documents.
The grading of discussions is also not difficult. Here is a link to Schoology's support page on how to go about grading discussion posts: Grading Discussion Posts
The link above allows for more easily finding all of a students discussion posts when grading. The video at right shows a very simple way to grade discussions.
The grading of tests/quizzes is one of my favorite features of using Schoology. This is because it has the ability to automatically grade many of the different types of test questions. Here is a link to Schoology's support page on grading tests/quizzes: Grading Tests/Quizzes
The video at the right and more info on this can also be found here: Creating Test: Process