It is important to

It requires use of the verb in its basic form rather than its normal tense form.

We don’t use the subjunctive very much in contemporary English unless we wish to sound very formal. With verbs like suggest, recommend, insist and adjectives like important, essential, imperative, crucial, vital, we often use should+infinitive instead of the subjunctive or we can use the normal tense form.

The reporting verbs and adjectives above are normally followed by a that-clause in which that itself is often omitted.

In your example, Olly, ‘It’s important that the lesson be funny’ sounds too formal.
We would normally say: ‘It’s important that the lesson should be funny.’

Compare also the following:

• The doctor recommended (that) he should give up smoking.

  It's important that he

• It's important that he
• Its importance or its importance
• Subjunctive grammar
• It is important sentence
• Subjunctive mood




• The doctor recommended (that) he give up smoking. (More formal)
  
• The doctor recommended (that) he gives up smoking. (Less formal)