I think every parent has spent some time hiding in a car crying, but I am glad you got out your frustration that way instead of spanking your daughter--that took a lot of self-control and you did both her and yourself a big favor. Spanking tends to increase aggression and anxiety, so you do not want to go down that road.
Sometimes problems that were there developing over time are revealed when we change something for the child--placing her in school is a perfect example. Often children who become aggressive after a new stressful event are highly anxious, and are becoming aggressive in a desperate attempt to maintain control. Little children who are this aggressive tend to be very unhappy, frustrated and anxious people. Their behaviors are happening for a reason, they are not random. For some reason your little girl does not have any better way she can think of to cope with feeling overwhelmed. Try to think of the behavior in those terms--that she is trying very hard to deal with strong emotions that are very upsetting to her.
I would recommend you seek the help of a psychologist who specializes in young children. First visit your pediatrician to rule-out anything medical that could be causing behavioral changes, and ask for a referral. The psychologist may recommend an assessment (see my Medhelp article on Developmental Assessment to learn more), or may recommend you begin therapy. The most effective therapy at this point will include significant time spent working with you, the parents. You and your husband can learn positive ways of helping her choose better coping behaviors than the ones she is using now. You can also improve your ability to understand things from your daughter's perspective and learn 'emotion coaching' (helping her identify, label and deal with her emotions). Ask for a clinician who uses Parent Mangement Training or Parent Effectiveness Training, but make sure you do not end up with a clinician who wants to work exclusively with your child without involving you in the process. Individual therapy for her alone is not likely to lead to much change.
In the meantime, check out these books: The Kazdin Method by Alan Kazdin (an excellent guide to positive behavior management) and How To Talk So Kids Will Listen and Listen so Kids Will Talk by Adele Faber and Elaine Mazlish (an amazing guide to preventing and reducing power struggles through improved communication). Good luck.
Disclaimer: This Medhelp post is written for informational purposes only. It is never intended to replace face to face psychological or medical care. This Medhelp post is not intended to create a patient-clinician relationship, nor to give or rule-out a diagnosis.
Have you ever heard of Love and Logic parenting? I have had an amazing experience with it. We were having some extremely stressful situations with our 4.5 year old son and the local elementary school was offering this class for parents. Check out the website loveqndlogic.com, there's even podcasts and lots of books. It will help you remain calm and in control while you handle your child in a very firm and loving way. Good luck!