Explanation:

- Option 1: "children are actually excited" is grammatically correct. The phrase expresses that children have enthusiasm for school.

- Option 2: "In the ideal 21st century classroom" is correct. This sets the context of the sentence by describing a futuristic school setting.

- Option 3: "there is little" has the error. The phrase should be "there are few" to correctly quantify "discipline problems." Discipline problems are countable, so "few" is appropriate.

- Option 4: "everyone is eager" is correct. The phrase effectively conveys that all individuals involved are keen to learn.

Correct Answer: Option 3 - "there is little"