Hi,

suppose every student has less than 12 friends. Pick any student A, he has less than or exactly 11 friends, so the number of students other than A who are not friends with A is greater or equal to 25-12 = 13, let student B be one of them. B has also less than or exactly 11 friends. Since 13-12=1>0, there must be at least one student C, other than A and B, who is not friends with A and is not friends with B. Since A and B are not friends with each other, A,B,C is a group of three students and no two of them are friends, which contradicts our assumption. Thus there must be a student who has at least 12 friends.