In that case you are likely going to have to go to court to get an order to see them. And you can also apply, on the same form referred to below for the child arrangements, for a specific issues order for the children to be returned to their school. You can make the specific issues order application as a matter of urgency and ask for the hearing to be within a day or two, as obviously time is of the essence to get them back to their previous school. The longer you leave it, the less likely a judge will order a return.
With regards ***** ***** child arrangements, if at all possible, the best option is to try to agree arrangements, with the help of a parenting plan if necessary. A cafcass leaflet and template for a parenting plan can be found here: https://www.cafcass.gov.uk/download/4365
If that doesn’t work, then the person who wants to go to court has to attend a MIAM, and if the other party engages, then mediation has to be tried before going to court. It is a legal requirement for anyone wanting to apply to court to attend a MIAM (mediation information and assessment meeting) prior to making their application to court. There are some exemptions to the need to attend, and those exemptions can be found listed on the court application form, so any applicant should check to see if they meet one of those exemptions. But for most parties, mediation will be required, and mediation can overall be the far cheaper and preferable way to resolve issues. The link to find a mediator is below.
If both the above fail, then court is the final resort. It really ought to be the final option and only used if absolutely no agreement can be reached. This is because court, frankly, tends to bring out the worst in people and often allegations come up which would not have been mentioned but for the court proceedings. Of course, if there is abuse in the relationship, court is the correct method of resolving issues as mediation will not be suitable.
The application for child arrangements is made on a C100, which is available in paper form and online, and the court fee is £232. If the applicant is on benefits/low income they may be eligible for fee remission and should see the link below for help with that.
The paper C100 application is here: https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/946762/c100-eng.pdf
The online C100 is here: https://www.gov.uk/looking-after-children-divorce/apply-for-court-order
Parties can find a mediator here: www.familymediationcouncil.org.uk
Fee remission assistance can be completed online, the system will give a code that is placed on the application form, if eligible. See here: https://www.gov.uk/get-help-with-court-fees
The applicant MUST attend the MIAM before applying as the mediator has to sign the C100 form - this is a legal requirement before they can apply to court.