Director of Public Health in Sheffield Greg Fell said although it is unknown how much it will go up, it is impossible to make it zero risk.
He said: "Our schools, not just in Sheffield, but up and down the country, have worked really, really hard to mitigate the risk.
"But it is not possible to make it zero risk. The epidemiological risk is the more contact humans have, the more chance for the virus to transmit."
He said the transmission between school staff is more prevalent compared to pupils, therefore, will lead to an upward infection when schools are fully back.
Mr Fell said the current declining rate was most likely due to the snowy weather that hit the city two to three weeks ago.
"People were reluctant to go out in the snow so fewer people were being tested, making the rate decline but it now appears to have leveled off.
"It will level off, it will go up, it will go down but at the moment it is going back down. I'm strongly suspecting it will go up again when the lockdown is easing starting next week."
He said although there is no single answer on whether the vaccination reduces the transmission of the virus, it is becoming clear that vaccinated people are less able to transmit the virus.
"More than 170,000 over people have been vaccinated and this will bring downward pressure on the infection rate. So, I think it will be alright for some time to come."
However, he cautioned that there will be a "bumpy ride" when the case rate is much higher than it was last summer.
Last summer, he said, the case rate was between 20 and 25 cases per 100,000 people compared to the current rate of 125 cases per 100,000 people.
"One of my huge concerns is unchecked or unmitigated threat of unvaccinated population, despite significant number of people have been given the jabs.
"But the spread of the virus by the unvaccinated people really worries me as more younger people are hospitalised. Secondly, the unmitigated spread that can cause long covid.”