System of education

As per ‘Taittiriya Upanishad’ education has only the following components basically:
1. Guru or teacher
2. Sishya or Disciple
3. Vidya or Education which is imparted
4. Pravachanam or discourse, Through which education is imparted.

what was the criteria?

Here there is no mention of from where the disciple is coming from or his economic criteria.
There is no mention of parents whether they are educated or not.
No mention of individual faith.
The only criteria is desire to learn.
Education was imparted in ancient india to any disciple who is a true seeker of education from any part of the earth.

Free education 

1. Education was free including boarding, lodging and clothes.
2. All expenditure was met by Governments and rich people.
Ancient Bharathadesam followed Free Education.
While disciples maintain celibacy and utmost respect to their teacher, teacher looked after disciples as own children.
Students were attracted from all over the world countries.
Knowledge spread everywhere.