There are a few aspects worth me touching on here. Absence due to sickness of the employee may be tolerated to different levels depending on the employer. Most have a policy stating the ‘triggers’ for what are deemed unacceptable absence levels ie after 3 days of separate absences or 5 days over say a year may invoke the disciplinary procedure in some cases, whereas other organisations may be more tolerant and have a trigger of, say, 10 days in one year before the employer addresses the absence issue with an employee. In cases where a doctors’ certificate is provided (and especially when employees have previously been reliable & have performed their duties to a high standard) companies may use their discretion to be very accommodating (even though it’s rare that organisations pay normal salary during longer term absence). So it will depend on the policy, your employment history to a degree and, of course, whether the absence procedure was followed during your absences (eg requesting time off in advance where possible etc).
Regarding your son’s surgery and your associated absence, there is a statutory regulation that allows you to take a ‘reasonable’ amount of unpaid leave in order to care for your child in case of emergency. ‘Reasonable’ is not defined in employment law so it’s difficult to advise fully not knowing the extent of your absences due to your son’s surgery. This unpaid leave is deemed to be available for short-term emergencies and also to enable the carer to arrange alternative childcare/care for the dependent. Normally an employer will have a meeting in advance of the situation becoming ‘unacceptable’ to try to agree a way forward and to give the employee ‘advance’ warning so they have an opportunity to resolve the problem/make alternative arrangements if possible. Parental Leave may also be a possibility (subject to certain qualifying conditions), if more than a couple of days is required to care for a child – this is unpaid too, and you can find out more via ACAS or the www.berr.gov.uk
In terms of whether you have a case for constructive dismissal, this again depends on certain qualifying conditions and also the circumstances of your case, the schools’ policy on absence etc etc. Having walked out, my first recommendation would be along the lines of arranging a meeting with your employer to explore the absence policy, and to then to consider whether you have a case for placing a grievance to try to resolve the issue amicably first of all. Whilst constructive dismissal claims may be successful depending on the circumstances of each case, there is normally a requirement to try to address the problem internally first of all – the Grievance policy / procedure would be the first port of call. Again ACAS can offer free advice – I would recommend you obtain a copy of any relevant school policies first, as well as your employment contract / conditions so that an Advisor can give you more specific advice based on your particular situation / circumstances.
Best of luck with coming to an agreement / resolution.