With the school closures lasting until at least autumn all teachers will be laid off. [ No they wont be , teachers remain in employment over the summer as they always have. You have no evidence of this and it is untrue as the basis for your statement]
Many will be forced to sign on and live off benefits. Some might take other jobs but if they “sign on” they will be committed to accept any job on offer or lose their benefit. Once they take another job some might enjoy it better salary and settle into it choosing not to return to teaching. [ No they wont be fired , so they wont be signing on. If you hadnt worked it out teachers are in demand, not only are there existing pupils to teach they have A levels and GCSE's to sort out plus they have to get ready for when school restarts]
Others may be trapped by the jobs contract and not be able to leave for 12-18 months meaning they can’t return to teaching if schools return before then. [ False employment contracts do not trap you for 12-18 months especially teaching, more fanciful nonsense by you]
Breach of job contracts can in extreme cases result in the employee being sued [ You dont know what you are talking about. Not for a teacher it wont. it rarely happens for other cases and they wouldnt be applicable for newly employed teachers].
Could this happen? [ No because you are misinformed on almost every point you make]
What will happen if we lose many teachers? [ You might lose teachers due to disenchantment with the profession, but not because the government sacks them]