Research just published by Professor Teresa McCormack, Dr Agnieszka Graham and Jessica Cherry from QUB has said that daydreaming "can be reliably measured in children and is of educational significance".

Their research carried out by the School of Psychology involved 97 children aged six to 11 years old and found that children spent a quarter of their time in class daydreaming.

The study concluded that if students fail to attend to instruction because of daydreaming, that it may affect their academic achievement in school
QUB study explores the effect of daydreaming in young - BBC News.

So is the old classroom maxim about paying attention right, or is there still a place for mind wandering for the bored pupil or stressed adult? According to the QUB research, studies involving adults show that higher rates of daydreaming, "have been implicated in poorer performance on a range of learning activities, including reading."

This view has support in academic publications, for example a 2010 Harvard study linked spacing out with unhappiness, concluding that "a wandering mind is an unhappy mind." Killingsworth and Gilbert suggest that their findings support the traditional view that; “.... philosophical and religious traditions teach that happiness is to be found by living in the moment, and practitioners are trained to resist mind wandering and to ‘be here now.’

There is a contrary view however. A typical example is set out in the reputable American medical journal Everyday Health, which argues that there can be positive aspects of daydreaming. For example they suggest that you can use daydreaming to your advantage by setting aside time each day for quiet reflection. The theory goes that setting aside a certain time each day when you sit in a quiet spot and begin daydreaming can be an ideal stress reliever after a long day of work.

The journal also suggests that daydreaming can be a stress-relieving technique after traumatic life events like arguments with loved ones or confrontation at work. It is proposed that daydreaming allows your mind to wander and forget about reality for a short time and that by allowing yourself to escape from a stressful situation, you can return to the situation with a new attitude and possibly even a solution to the problem that may be causing the stress.

At a simpler level the journal supports the view that daydreaming can also be a way to relax. It allows your mind to take a break from challenging realities and return with a refreshed and renewed mind.

Another advantage is possibly that daydreaming can help you concentrate and focus on future life goals. Thinking about goals and focusing on the steps to achieving that goal make it more likely that you will be to reach it.
Positive and Negative Effects of Daydreaming | Everyday Health

One radical new theory of human intelligence argues that having your head in the clouds might help us with the activities that are most important to us. According to Scott Barry Kaufman, NYU psychology professor and author of Ungifted: Intelligence Redefined, we need a new definition of intelligence -- one that factors in our deepest dreams and desires.

"We all have goals and dreams in life -- things we want to accomplish,

.......while the kinds of skills that are measured on IQ tests are important ... there are so many more characteristics  helping us to reach those dreams and goals."

Kaufman suggests that our traditional definition of intelligence may be lacking and fails to include many people who don't perform well on rote cognitive skill tests.
How Daydreaming Can Actually Make You Smarter | HuffPost UK Wellness (huffingtonpost.co.uk)

Are we therefore suggesting that our current classroom conventions are biased in favour of pupils with the kind of intelligence that does well with rote learning and that equally intelligent pupils who need to process information in a different way are at a serious disadvantage? There are countless examples of great and gifted people who have not reached their full potential until they escaped the education system, so perhaps we need to pause and consider whether this new research highlights just the reality that many children daydream but also whether the harm to their learning is compensated by insulation from harmful stress and/or space for thinking creatively.

 The QUB study found that..

"Mind wandering is detrimental during educationally significant activities," and that;

...in an educational context, if students fail to attend to instruction because of (daydreaming) this may impede their chances of acquiring crucial skills or knowledge.”

 Following the release of the study, Dr Agnieszka Graham  has proposed that finding out more about when and why children daydreamed in school could help teachers find ways to reduce it in class. This suggestion certainly encourages teachers to try and make lessons more stimulating and interesting to avoid triggering the daydreaming which comes with loss of interest. We are left however with the other dimension of daydreaming, that it might be an important safeguard to mental health and an essential coping mechanism for dealing with difficulty.

Mental Health helplines like Verywellmind.com suggest mind wandering as an option at our disposal which can be useful especially when we deal with perceived threats or overly busy environments. They describe it as another tool in your mental health toolkit to evade stress and anxiety. They suggest that if you feel yourself getting more and more anxious, you might turn to daydreaming. The following steps are suggested;  to.. look away from your desk, your work, or any distractions,  breathe in deeply, then breathe out slowly and repeat and lastly, think of something pleasant that has meaning to you.
5 Positive Effects of Daydreaming (verywellmind.com)

This advice is typical of common sense advice on good mental health. The overall message regarding mind wandering and daydreaming in school seems to be that like so many good things in life, it can be really beneficial to happiness and well- being, but that used to excess it can become a real problem with serious downsides.