Loomis is right about the proportions; that being said I looked at your sketchbook and saw that you have not drawn the proportions correctly as Loomis has laid them out, so your teacher is right as well.
Take this image for example:
http://www.conceptart.org/forums/att...1&d=1335207242
You have made the distance from the bottom of the chin to the bottom of the nose two units. THe distance you made from the bottom of the nose to the brow line is then three units, and the distance from there to the hair line is roughly one and a half units. The eyes are also too high on the overall head--they should lie roughly half way up the head from top to bottom.
Some of your other faces in your sketchbook fit the correct proportions more closely, but some are also way off. I'm wondering if on some of them you are using the incorrect points to measure from. The Loomis method uses the bottom of the chin to the bottom of the nose as one unit, the bottom of the nose to the brow line (a point roughly where a unibrow would be, it is based on the bone structure beneath) as another equal unit, and the last equal unit is from the brow line to the hairline (which is the most variable since hairlines and foreheads change person to person, and hairlines recede with age). In some of the portraits you have drawn, the point you have defined as the brow line is not correct (you place it too low), and this will make the nose too long as your teacher states.
Keep in mind that not every face will perfectly fit Loomis' model. For someone beginning out like you he is a good start and will set a general model in your head to work from, but people's faces vary, will change with age, and you may want to change things ever so slightly anyhow to better capture an expression or particular face. Most faces however fall fairly close to what Loomis has shown in his proportions, so learn them well and be able to recognize when a face is drawn too extreme and is not natural.
Hope that helps!
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