Antivirus Solution for K-12 education at no cost.

Wee have been struggling to allocate a budget in the past few months and due to this we are looking for a potential antivirus solution ( the whole school is currenty using Windows Defender on WIN10) which is not really safe.  We tried to get in touch with Avast,F-Secure or Bitdefender and few more but none of these had free of charge AV solutions.

Does anyone of you know a solution that offers free of charge for K-12 schools?

I would really appreciate it