That Twilight Zone episode was based on Examination Day by Henry Slesar.
"The Jordans never spoke of the exam, not until their son, Dickie, was twelve years old. It was on his birthday that Mrs Jordan first mentioned the subject in his presence, and the anxious manner of her speech caused her husband to answer sharply.
‘Forget about it,’ he said. ‘He’ll do all right.’
They were at breakfast table, and the boy looked up from his plate curiously. He was an alert-eyed youngster with flat blond hair and a quick, nervous manner. He didn’t understand what the sudden tension was about, but he did know that today was his birthday, and he wanted harmony above all. Somewhere in the little apartment there were wrapped, beribboned packages waiting to be opened, and in the tiny wall-kitchen something warm and sweet was being prepared in the automatic stove. He wanted the day to be happy, and the moistness of his mother’s eyes, the scowl on his father’s face, spoiled the mood of fluttering expectation with which he had greeted the morning.
‘What exam?’ he asked.
His mother looked at the tablecloth. ‘It’s just a sort of Government Intelligence test they give children at the age of twelve. You’ll be taking it next week. It’s nothing to worry about.’
‘You mean a test like in school?’
‘Something like that,’ his father said, getting up from the table. ‘Go and read your comics, Dickie.’ The boy rose and wandered towards that part of the living room which had been ‘his’ corner since infancy. He fingered the topmost comic of the stack, but seemed uninterested in the colourful squares of fast-paced action. He wandered towards the window, and peered gloomily at the veil of mist that shrouded the glass.
‘Why did it have to rain today?’ he said. ‘Why couldn’t it rain tomorrow?’
His father, now slumped into an armchair with the Government newspaper rattled the sheets in vexation. ‘Because it just did, that’s all. Rain makes the grass grow.’
‘Because it does, that’s all.’
Dickie puckered his brow. ‘What makes it green, though? The grass?’
‘Nobody knows,’ his father snapped, then immediately regretted his abruptness...."