Being in the "middle" is not a class. Social classes - certainly in Marxist theory, but also in other classes analyses of society - are not the rungs on a ladder: Low, Middle, High. Rather, they are characterized by their social role (e.g. role with respect to production, distribution, ownership etc). And a social class may have more or less power and wealth, depending on circumstances.

Centuries ago, people who owned land, workshops etc. but were not nobles were referred to as the "middle class" - the middle between the peasantry and the nobility. Today that moniker is meaningless, though it persists - even though people of "middle" wealth are in no way not a distinct class.