She can now challenge the points he makes and in some instances she proves him wrong. She did lack confidence in her own ability and therefore she didn't invest the time OR attention she needed to. She was willing to accept his viewpoints as being correct, and she never dared to question them. One example of when she proves him wrong with her newly-found ability and confidence is when Frank makes a point about Blake, and Rita argues with him, and she goes as far as correcting him.
The attitude towards class and culture has changed greatly for Rita. She has come into contact with an entirely new culture. Her viewpoints about the upper-class people she so badly misunderstood before are changed, and she knows what it's like to be "one of them", something that Rita has wanted all along. That was her goal, to be accepted.
Rita's attitude towards Frank's study is that it is now stuffy and suffocating. She refers to the room as a plant, and that it needs airing. The jammed window in the scene represents the restrictive atmosphere of the study. Rita's new "positive" attitude that she gained from Summer school is the reason she's telling Frank these things, she's happier in herself and sunshine reflects happiness.
Another part of Rita that's changed after Summer school is her attitude towards the future. Now she has accomplished everything she wants - to be accepted as "clever". She realises the opportunities that she now has for the future, and already did have. She also recognises that her family had made her feel that the age of 26 was too old for her - even though they were wrong, she"d heard it so much that she started to believe them. She knows now, after the experience of summer school that that couldn't be further from the truth, it's never too late to achieve and we"re always growing and developing.
Willy Russell used a number of devices throughout to get his point across.