http://dev.mysql.com/doc/refman /5.0/en/insert-select.html
Per il caso1:
INSERT INTO TAB_STUDENT(name_student, id_teacher_fk)
SELECT 'Joe The Student', id_teacher
FROM TAB_TEACHER
WHERE name_teacher = 'Professor Jack'
LIMIT 1
Per case2 devi solo fare 2 istruzioni di inserimento separate