Absolutely unreasonable.
Secondary teachers will have difficulty learning who their pupils are and building up a rapport. It will already be harder from not having seen pupils since March.
Many teachers and pupils will struggle to communicate effectively through muffled speech and loss of significant chunks of non-verbal communication. I used to teach in a school with enhanced provision for hearing impairments, and occasionally had no cover if the BSL interpreter was unavaliable. Lip reading and expression go a very long way in effective communication and not just for people with recognised hearing and processing difficulties.
Pupils with additional needs/ special needs are not snowflakes. Be it misted up glasses, asthma, ASD, ASHD, hearing impairments as some common conditions that make mask wearing impractical for a sifnificant core in every class.
For a core of pupils, it will be a fantastic excuse for low level disruption to be difficult to pin down and deal with. Some classes/ schools have a very difficult core that do not care about disrupting learning or the consequences to them. So much scope to waste even more precious learning time working out who's whispering/ humming or flinging their "disease ridden" mask around the room for the ewww factor. Then there's just the general fidgeting and faffing.
Plus the fact that the type of masks are ineffective for long term use in a closed environment and once the mask is touched any virus transfer is now onto hands and the physical environment. For very little evidenced gain, there is a significant detriment to the teaching and learning experience and will be felt most strongly by those who have been most left behind for nearly 6 months.