Under this system, students must take turns teaching each other and helping one another.
Literal
This system [possessive-の] under [at-で] [topic-は], students [topic-は] alternation [in-で] teach-each-other, mutually help-each-other-must.
~合う is a reciprocal suffix: 教え合う (teach each other), 助け合う (help each other). ~のもとで means 'under' in the sense of operating within a system or framework. 交替で uses で to mark the manner — 'by taking turns.' The obligation ~なければなりません emphasizes the systemic requirement.