Except/except for

[Rustamsher]

1a. Except Bobur all the invitees turned up.
1b. Except for Bobur all the invitees turned up.
2a. Except the C in geometry, he made all A's this term.
2b. Except for the C in geometry, he made all A's this term.
3a. Nobody helped me except you.
3b. Nobody helped me except for you.

Which of the sentences above is/are grammatically incorrect?

Thanks.

[Alan]

1b, 2b, 3a, 3b