Set the highest grade as "100%". This is one of the most common (if not the most common) methods teachers and professors use for curving grades. This curving method requires the teacher to find the highest score in the class and set this as the "new" 100% for the assignment. This means that you subtract the highest score in the class from the hypothetical "perfect" score, then add the difference to every assignment, including the highest-scoring one. If done correctly, the highest-scoring assignment will now have a perfect score and every other assignment will have a higher score than it previously did.
For example, let's say the highest grade on a test was a 95%. In this case, because 100-95 = 5, we would add 5 percentage points to all of the student grades. This makes the 95% score an adjusted 100%, and every other score 5 percentage points higher than it was.
This method also works using absolute scores, rather than percentages. If the highest grade was a 28/30, for instance, you would add 2 points to the score of every assignment.