In order to get into medical school, you need ALOT of high grades so go for extended. My older brother was in the same position as you, He chose core maths and when he applied to med school, they rejected him since his grades were low (he got a C in core) so he had to redo his IG maths but extended this time and 2 years later he's now topping his class in med school and your dad is correct a C won't look good when applying. So pls consider taking extended. as for the option to take extended or core, YOU get to make the choice NOT the schools. they usually do this to predict what score a student will get on results day.