One word that can be used to represent Weak student?

Alternate terms for "weak student" are "underachiever" (consider this Ngram), and "underachieving/low achieving student."

For your example, consider "lower achieving students."

The heuristic model gives higher achieving students an opportunity to find possible solutions, whereas lower achieving ones are ...

Consider also "lower performing students," "low scoring students," and "low graded students":

The heuristic model gives higher performing students an opportunity to find possible solutions, whereas lower performing ones are ...

The heuristic model gives higher scoring students an opportunity to find possible solutions, whereas lower scoring ones are ...

The heuristic model gives higher graded students an opportunity to find possible solutions, whereas lower graded ones are ...

The term struggling learner, or struggling student, is often used to describe a student who has difficulty in keeping pace with their classmates or expected learning benchmarks in a developmentally appropriate environment. They are separate from students who may have special needs or who have been identified as at-risk.

nonachiever (or non-achiever)

a student who fares poorly in the classroom or has failing grades.

Other than that, "weak student" is correct also and more common.

weak: Lacking aptitude or skill: a weak student; weak in math.

Dunce, but probably not in this context ;)