Authors' contributions statement to highlight contribution of an undergraduate student
In mathematics, the default assumption is that all authors contributed more or less equally to a project, so the main reason I see to describe individual author's contributions in a mathematics paper is that the authors want to attribute credit of certain aspects of the paper to individual authors.  (This can also be done with appendices by subsets of coauthors.)

It does not make particular sense to me to highlight someone's contributions just because they are a student. You are free to do so if you wish, but it will be weird if you only describe one of the authors' contributions.


  
  To avoid opposition from coauthors who may question listing a student as an author. 
  


This is completely ridiculous.  I have never heard of any co-author complaining that someone should not be listed as a co-author just because they are a student.