YI'm a teacher and have to collect my students' phones on a regular basis. I'd say about 90% of all the phones I collect are iPhones. However, of those 90%, at least 80% are iPhone 4 models and even some 3GSs. Apple used to be popular but the newer models don't sell at all among younger people.
If 90% are iPhones, then that does not prove that it is losing popularity because your students not switching to android or WP. Parents can't buy kids new phones every other year. iPhone 4/4s is still a good or usable phone although a 3GS might not be the phone it used to be. Judging from your comment Switzerland is among the earliest adopters. Japan's iPhone boom really took off only after the 5 launch so everyone has a relatively new model here. My friends who were early adopter is still clinging on to their 4/4s.
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For example my colleague has his quad-core Android plugged in all day because the battery (which he'll always boast is 4x the size of mine... so is 'better') becomes drained within ~3 hours of usage. His camera takes high MP, useless quality photos... which he can view on a lower resolution but physically larger screen (which doesn't fit into his pockets so has to be carried in his hand or a man bag).
What phone is that? (So I can tell my android friends not to buy that model