When attend means ‘to be present at an event or activity’, it is not used with the prepositions to or at. It is simply followed by a direct object:
✗ You go to university, attend to classes, but learn nothing about the real world.
✓ You go to university, attend classes, but learn nothing about the real world.
✗ They are able to attend at important meetings.
✓ They are able to attend important meetings.
In the same way, when attend means ‘to go regularly to school, church etc’, use a direct object (not attend to or attend at):
✗ In some parts of our country, people still don’t let girls attend to school.
✓ In some parts of our country, people still don’t let girls attend school.
✗ …a peak in the number of students attending at universities in Sweden.
✓ …a peak in the number of students attending universities in Sweden.
Don’t confuse these meanings with the phrasal verb attend to something, which means ‘to deal with something’:
My assistant will attend to all your travel arrangements.