Below is the list of tables from the database along with their columns and datatype. Need to generate an accurate SQL query for the question.
Only use the columns present in the respective table. Only use the columns which are required to generate the SQL query. Only use table joins if required and not otherwise.

### Tables ###
TABLE: CREATE TABLE Highschooler (id VARCHAR)
TABLE: CREATE TABLE Friend (student_id VARCHAR)
TABLE: CREATE TABLE Highschooler (grade INTEGER, id VARCHAR)

### Question ###
Find the average grade of all students who have some friends.

### Accurate SQL ###

SELECT AVG(grade) FROM Highschooler WHERE id IN (SELECT T1.student_id FROM Friend AS T1 JOIN Highschooler AS T2 ON T1.student_id = T2.id)