You don't say how old your boys are. If they are 2 year olds my advice would be different than if they were 4 year olds. That aside, most kids outgrow this stage very quickly, especially if the preschool they're attending is staffed by caring people and lots of fun things to do!
It sounds like your boys are experiencing some separation anxiety. That's very common with preschoolers who've not been away from Mom before. But if you don't feel absolutely confident in the school's ability to take care of your boys while you're away then it is possible that your sons are picking up on YOUR separation anxiety. If they see you tense up each morning or if you are overly concerned about their crying at night and pay a lot of attention to their "I don't want to go to school" comments, then the message you may be giving them is "School is NOT a safe place."
But if, on the other hand, you can reassure them that "School is FUN" and that you'll be back for them at 12 o'clock. (Make sure you're there a bit early... never late.)
One other thing... good preschool teachers are compassionate wonderful people and can be very helpful during teary goodbyes. So don't worry... your sons will outgrow this and learn to look forward to their time with friends at school.
I'm sorry to hear that you've encountered this challenge with your boys. I suggest discussing it with their pediatrician and/or a family counselor, and learning more about why they are having this reaction about going to school. Was there a traumatic incident at one of their previous preschools? Any bullying? A doctor, family counselor or child development specialist may be able to help you explore this further, and help you learn if there is something about going to preschool that they haven't been yet able to communicate to you.
Below are some additional informational resources you may also find helpful. Thanks for asking, and all the best to your family!