The teacher looked at the misbehaving student sternly before addressing the class.
I can give you several sentences.He spoke sternly to the misbehaving students.I backed down when he glared sternly at me."Follow your orders," the sergeant said sternly.
The teacher looked sternly at the students who were misbehaving in class.
Firm or unyielding, uncompromising.
Lamented or sternly
They are synonyms, not antonyms.
Rigorous.
Meaning hard, harsh, or severe in manner or character:
Sternly disciplined, brave, undaunted
"Sternly" means in a strict and serious manner, often showing disapproval or authority. It is used to describe someone's tone or demeanor when they are being firm or demanding.
grimly, sternly, austerely, seriously, solemnly, thoughtfully
Look at them very, VERY sternly