score:0

The span is a sibling of the checked input, not a descendant.

See: Adjacent Sibling Combinator

.buttons {
  padding: 30px;
}

.buttons input:checked+span {
  color: blue;
}
<section class="buttons">
  <label>
      <input type="radio" name="options" id="option1" checked />
      <span>9th grade</span>
  </label>
  <label>
      <input type="radio" name="options" id="option2">
      <span>10th grade</span>
  </label>
  <label>
      <input type="radio" name="options" id="option3"/>
      <span>11th grade</span>
  </label>
  <label>
      <input type="radio" name="options" id="option4" />
      <span>12th grade</span>
  </label>
</section>


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