Appearances/Looks can be deceptive/deceiving.

MEANING
people or things are not always the way they look

EXAMPLE SENTENCES:

The pupils looked angelic – but appearances can be deceptive.

He appears to be happy, but appearances can be deceptive/deceiving.

Appearances can be deceptive.

Looks can be deceptive.

Appearances can be very deceiving. [ =deceptive]

Gwen’s students may look angelic, but appearances can be deceptive.