As mentioned earlier which is more important? The kilt or the job.

I believe the issue is in her mind and as the administration has told you that the School (Employer) will not tell you what you can and cannot wear, then you have no problem as far as keeping a job.

What's important now is ensuring that she cannot ever argue that you have slighted her in service or made her feel threatened because of her issue with your kilt.

The best would be to come in, fix her computer, and leave. Always being professional and corteous. Maybe even keep a detailed journal of her service calls in case you ever need to recall something that happens.