After a heated conversation with my husband (he’s an executive headteacher of two outstanding schools), we agreed that of all the attributes required for leadership, honesty is the most important.

Honesty was the key driver behind the transparent assessment system I introduced when I was principal at Burlington Danes Academy in West London. I wanted to lift the veil on student achievement by being candid with pupils and their parents if they were failing to meet expectations. Similarly, honesty was the bedrock of my relationship with staff.

The latter was arguably more difficult than the former. A key part of being honest with staff is constructive criticism and that can be tricky to deliver.

I want to be clear that the vast majority of my conversations with my colleagues are full of praise and positivity. Most teachers are talented, motivated professionals who want to learn and help students to learn. In all my headships, I’ve sought to establish a warm and supportive culture. For example, I’m proud to say that tea and biscuits were served in the staffroom at Burlington Danes every day, and retaining and developing teachers was one of my top priorities throughout my tenure.

However, constructive feedback is an inevitable part of the job. Here’s how I handle it.

Consider the problem

There’s no easy answer to the question of when to challenge underperformance, but when grappling with this issue I remind myself that fairness matters as much to teachers as it does to students. Teachers get fed up when they have to cover for absent colleagues or when people in their departments aren’t pulling their weight. It must be clear that the headteacher means it when they tell a teacher that they have done something well - and the flip side of this is that poor performance must always be tackled, not tolerated. In the long run, teachers will thank you for not avoiding tough talks about performance.

Make it private

The biggest difference between my praise and criticism of staff is that I have always made sure that the former takes place in public but the latter in the privacy of my office.

Collect evidence

Difficult conversations begin with gathering sound evidence. If you’re going to challenge teachers on their performance, you have a duty to form a complete picture of the job they are doing. This will be based on student data, lesson observations, performance appraisals and, of course, the views of the teacher’s line manager.

There’s a real danger as a headteacher of making judgements on poor evidence. Teaching is a performance profession and it’s easy to be taken in by a bit of showmanship in the classroom. At the other end of the spectrum, it’s easy to ignore the efforts of the teachers who just get on with their jobs, avoiding the limelight and delivering great lessons day in, day out.

The antidote to this problem is creating a school that is rich in data. It’s easier to tackle poor student progress when you have the data to compare one teacher’s efforts with the rest of his or her department.

Rehearse first

After gathering the facts, I prepare and rehearse what I’m going to say. Great teachers go into their lessons with a script of critical questions, and it’s the same with some of my toughest conversations.

Get to the point

Once the chat is under way, I get straight to the point and try to drill down to the precise issue causing concern, whether it’s shoddy planning, lax marking or questionable teaching. In the same way as I don’t speak to the whole staff about marking books if only a few teachers are at fault, I won’t speak to a teacher about their whole practice if the only issue is that they don’t set homework.

Point the way forward

Crucially, a headteacher must always provide a pathway for improvement. Whether it is coaching, mentoring or a change of role, genuine time and support must be given for the agreed issue to be addressed.

Follow up

One final piece of advice: you have to give colleagues a bit of time to get back to you once you’ve talked. I often find that a teacher who appears to accept criticism in the first instance will come back the next day with a different opinion, so I make a follow-up appointment before finishing the first meeting.

These difficult conversations have become easier for me over time because my feelings about what children need have strengthened. I’m more passionate than ever about the importance of every lesson being led by a talented, dedicated teacher, so by focusing on the outcomes for the students, I manage to get my point across.

Dame Sally Coates is director of secondary academies (south) for United Learning and the author of Headstrong: 11 lessons of school leadership