Why grades from 0% to 100% considered discrete? It can be 67.95% which means it's not an integer.

The percentage is discrete or continuous based on the data whose percentage is being calculated. If the percentage of discrete is being calculated then, the percentage will be discrete, and if the percentage of continuous is being calculated then continuous. In above question, since the marks given to any student will be discrete and total marks will also be discrete. So, the percentage will be discrete.

Maybe the context is the multiple choices questions with only one answer

discreate here as my understanding for the grade it is a specific value that doesn't change with time so it can be plotted by a single point on the graph

I guess once you receive your grades they remain the same so they might be called discrete but Height and other function changes(height because in the morning we are an inch or two later till evening our height changes back to normal( or what we think) although these are very minute(very small) but still continuously changes)