6 year old with a bladder control problem?

My daughter is also 6 and she has this same problem. I have taken her to the doctor and there is not a problem. It usually happens when she is playing "she just holds it until she can't hold it anymore". Or it happens when she laughs :O) She will say "oh no i wet myself a little"...she just started 1st grade last week and on the 3rd day she had an accident BUT she did tell her recess teacher that she had to go and the teacher told her she would have to wait... but once she has to go she has to go RIGHT then, she has already held it to the point that it is right there and ready. It is just something that we have began to work on. I tell her to go and use the bathroom at every chance she gets "while at school" even if she does not "have" to go she has to try. Then I called the teacher and told her that if she has to use the restroom I want her to be allowed to go right then, that she can not hold it. She is getting better, I think that it is just a matter of taking time out of whatever she is doing to use the bathroom, she is a little girl and precious and if this is the worst thing that we face this year we are blessed.