Hi, and welcome to the forum.

I know Rouen best as a rather nice town we pass through on our way from the Paris area out to Normandy. The official town website is here: Actu • aujourd’hui | rouen.fr (it's all in French) and I know they have a pretty active AVF (a sort of welcome group for newcomers) here: Accueil Ville France Rouen (also all in French).

I assume what you refer to as a "medium school" is middle school or junior high, in which case what you're looking for in the French system is a "collège" which covers roughly the same age range (i.e. pre-high school).

Some French public schools may have an "English section" where some or all of the classes are conducted in English - though the curriculum is strictly according to the French national curriculum. These sections are free, though there is an admission process in which they try to balance the proportion of native English speaking children with those who are learning the language. (i.e. a native speaking child may well have a better chance getting in, but not always). Otherwise, you'll have to find a private school that teaches in English. A good source for English schools in France is the AAWE Guide to Education available from the association AAWE - Guide to Education, Vital Issues, AAWE News, AAWE Directory - however, like most clubs and associations in France, the AAWE offices are closed right now, until September 1st.
Cheers,
Bev