Problem 672

4.7.

Proof: 7 people are all classmates.
Take any student $A$, according to the problem, among the remaining 6 people, there are at least 3 classmates, denoted as $A_{1} 、 A_{2} 、 A_{3}$.

Next, we prove: the remaining 3 people $B_{1} 、 B_{2} 、 B_{3}$ are also classmates of $A$.

If not, there must exist $B_{i}(1 \leqslant i \leqslant 3)$ who is not a classmate of $A$, then, $A_{1} 、 A_{2} 、 A_{3}$ are also not classmates of $B_{i}$, which means $B_{i}$ has at most 2 classmates, contradicting the condition that "each person has at least 3 classmates".
Therefore, the 7 people are all classmates.