Yes it's perfectly possible. I passed my ABRSM ATCL without a teacher. It does however require serious dedication and you need to understand how you play very well. As without the help of a teacher to guide you through technical difficulties, they can present challenging obstacles to overcome.
But if you know how to practice and use musical friends as a feedback and improvement tool, more than possible to achieve a very high level of proficiency without using a teacher.
As has been said, record yourself, and be ridiculously strict on the tiny little things (much like a teacher is) and you will progress well. However only record yourself once every 2 weeks. So record piece, find all the tiny things that weren't quite right, work on them. Record again in another 2 weeks, check progress. Recording to frequently can become detrimental.