upper grade students tu___ lower grade student

The phrase to fill in is "tu" which typically implies a relationship of teachers to students. "Upper grade students tutor lower grade students" is a common phrase indicating that older or more advanced students are helping or teaching younger or less experienced students.

The correct word to complete the phrase is "tutor." In this context, "tutor" is a verb that describes the action of assisting, guiding, or teaching someone who is learning something new. Upper grade students often have more knowledge and skills, making them suitable to help lower grade students with their studies, thus fostering a collaborative learning environment beneficial to both parties involved.