Okay I am a beginning band teacher and unfortunately Oboe is not one of my main instruments. I have two students that picked up oboe pretty fast but are having trouble with some notes (Staff D on up). So I was wondering what notes are the hardest to get out on the oboe so that I may study up on it.

i would say the hardest notes are above a high C and its really hard to tune a C to in my opinion the d flats are hard to get out to and there are 3 alternate F keys to

Here are a few reasons I can think of off the top of my head.
i)if the oboes are in poor adjustment that provides the instrument with added resistence to blow against and creates some trouble for students.
ii)are the students using the proper octave keys and fully covering the half hole for the notes that do not require it, such as E on the top space and up?
the other possibility is the way in which they are blowing.
You need a focused and fast air stream as opposed to a voluminous air stream. If you can get your students to imagine trying to blow out a birthday candle at arm`s length that may help. They soon realize that they need a fast well-directed wind stream to accomplish this.
Hope something here helps.

If they`re having trouble with everything from staff D on up, that indicates not just what everyone else has said above but probably a poorly developed embouchure. It may sound counterintuitive, but having them do a lot of long tones in the range where they CAN play will really help them develop the embouchure to start tackling the "higher" notes.

As a general reference point, D and D flat and E on the staff are notoriously hard-blowing notes. They`ll have a much easier time adjusting to playing G or F, which are both fairly easy-blowing. It may help to have them transition from playing the lower G or F then just putting on the octave key to see if that`ll help them get the note.

You might also want to check that their reeds are sufficiently soft (medium soft, preferably not a store-bought) and not TOO soft (a true `soft` might be closing up in their mouth inadvertently because of the airstream pressure of the higher note).

Jn4Jenny- I just needed to pay you a compliment. You explain things very well and will be an excellent teacher someday. I go to the oberlin conservatory of Music and am a student of the virtuosic James Caldwell, if you ever want to chat you can email me at oboelinadam@yahoo.com and i will provide you with my sn. Best of luck in your oboe studies.

Adam Parnell