Just guessing based on what you have written, but generally 'expat' conditions - the ability to deduct housing, school fees, pay into home country social security rather than into Swiss, take international health insurance rather than Swiss, etc - are granted for a maximum of 5 years. After which an employee reverts to 'local' status and faces the same tax scheme as everyone else.
If you have been here 6 years this is likely what you are seeing. After your 5th year elapsed, which sounds like 2014, you are no longer eligible for 'expat' tax treatment.
This special tax regime was designed to entice highly qualified temporary staff here, those that are going to come in, do a specialized job no one else can do, and then leave, having brought local staff up to speed. Note 'temporary. After five years it is assumed that you are no longer temporary, have decided to make Switzerland your home - and so there is no need to offer further concessions.
We came here on an expat package, then after 5 years transitioned to a local one. As have many, many others. The decision to stay or go depends on how hard you need to bite the bullet and whether staying in Switzerland offers other intangible benefits that make the financial cut worth it. We knew all along that expat benefits were limited to 5 years - it was clear in our negotiations, and reinforced by all the research I have done over the years into tax matters.
We do not have children, but those I know in similar situations who do all moved their children into local schools once they decided to stay - most moved their children after the second or third year, in fact.
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Do be aware that Switzerland generally functions on the 'selber schuld' mentality. That is, one ought to know X, Y, or Z - and if one failed to find that information, it's one's own fault if one is tripped up. Whether one agrees with that attitude or not one must understand that it exists before one proceeds down the road of claiming negligence.
Being realistic - suing your employer is a very quick way to find yourself unemployed, especially in Switzerland where a reason for dismissal is not needed. Do you really want to go down that road? I'd think very carefully here.
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You should sit down with a tax professional, go over your situation and make plans from there. You might find that there are other tax planning strategies that could be of benefit to you.
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Do you want to stay in Switzerland?
Can your children transition to local schools?
If your children cannot transition to local schools and you cannot afford international schools on your own, perhaps it is time to move on.
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Of course, all of the above is my assumption based on the 5 year rule. There could be something entirely different at play here and I could be wrong. So re-read your contract, speak to the cantonal Steueramt yourself for an explanation.
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Wishing you all the best...