According to the stem analysis, 78 ~ 95 * * has 18 different scores, and these 18 scores are taken as 18 drawers.

Assuming that there are three people with the same grade in each drawer, there are 18×3=54 people.

At this time, if there is one more person, there will be at least four people with the same grades in a drawer.

That is 54+ 1=55 (person)

A: At least 55 students should be randomly selected in this class to ensure that 4 students with the same grades can be selected.

So the answer is: 55.