Pupils could be excluded for "malicious coughing" or making "inappropriate" jokes about the coronavirus pandemic, some schools have warned.
The new behavioural guidelines have been issued to parents as many schools prepare to welcome children back into their classrooms in England this week.
It comes as the Department for Education (DfE) said teachers could see an increase in bad behaviour due to pupils' lack of regular attendance and "classroom discipline" during the pandemic.
The Ark Alexandra Academy in Hastings, east Sussex, has set out a list of "coronavirus red lines" which will result in fixed-term exclusions for pupils should they be breached.
These include "deliberate or malicious" coughs or sneezes, "humorous, inappropriate comments or statements" related to Covid-19 and "purposeful physical contact with any other person".
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In a letter to parents in August, Jerome Scafe, network associate principal, said: "Any student that needs to have a fixed-term exclusion during the pandemic, will not return to main circulation until a risk assessment and we can be assured that the student will adhere to all our expectations."
Meanwhile, Ark Byron Primary Academy in Acton, west London, said in its letter to parents that if a pupil refuses to follow hygiene routines and social distancing instructions they will "immediately be moved to a separate area".
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It said: "Some behaviours (eg coughing deliberately on another person) that were previously 'simply' anti-social, are now potentially extremely serious."
The DfE said schools should clearly state the consequences for bad behaviour, particularly around new movement restrictions and hygiene rules.
In its updated guidance for schools, the DfE said on Friday: "It is likely that adverse experiences or lack of routines of regular attendance and classroom discipline may contribute to disengagement with education upon return to school, resulting in increased incidence of poor behaviour."
The DfE added that schools should work with pupils "who may struggle to re-engage" by providing them with support "for overcoming barriers to attendance and behaviour and to help them re-integrate back into school life".
