2 problems when trying to create a trigger in MySQL

Question

I am trying to understand triggers in MySQL, but am having a few problems.

I'm trying to implement a trigger which on every UPDATE/INSERT in the table Grades it updates the column "gpa" in another table called Student, but cannot do it properly.

Code:

CREATE TABLE Student 
(
    Id INT NOT NULL AUTO_INCREMENT PRIMARY KEY,
    name VARCHAR(30),
    age TINYINT,
    gpa NUMERIC(3, 2) DEFAULT 2
);

CREATE TABLE Grades
(
    StudentId INT PRIMARY KEY,
    grade_bg INT,
    grade_math INT,
    grade_subd INT,
    FOREIGN KEY(StudentId) references Student(Id)
);

delimiter |

CREATE TRIGGER update_gpa
    AFTER INSERT
    ON Grades
    FOR EACH ROW
    BEGIN
        UPDATE Student SET gpa = ((grade_bg + grade_math + grade_subd)/3) WHERE StudentId = Id;
    END;
|  

After this when I try to insert in the table Student I get: "Error 1054: Unknown column 'StudentId' in where clause".

For example:

INSERT INTO Student(name, age) 
            VALUES ('Joshua', 17);

Also when I try writing "AFTER INSERT, UPDATE" I get a syntax error from the MySQL Workbench and don't know how to make the trigger activate on INSERT AND UPDATE in the table Grades.

Any help is appreciated! Thanks in advance!