When validating the instance document http://home.arcor.de/martin.honnen/xml/oneCharInstance1.xml against the schema http://home.arcor.de/martin.honnen/xml/oneCharSchema1.xsd Xerces reports the following validation error(s):
"[Error] oneCharInstance1.xml:3:25: cvc-length-valid: Value '?' with length = '2'
is not facet-valid with respect to length '1' for type 'one-char'.
[Error] oneCharInstance1.xml:3:25: cvc-type.3.1.3: The value '?' of element 'test' is not valid."
The "test" element however contains a single character (<test>𐌀</test>), albeit one which is not inside the BMP. In terms of the XML specification http://www.w3.org/TR/xml/#dt-character and the schema data type specification http://www.w3.org/TR/xmlschema-2/#string there is no difference between characters in the BMP and outside of it, each one counts as a single character.
So the sample XML is valid against the sample schema and Xerces should not report any error.
Other validating parsers like Saxon 9.4 EE and XSV (http://www.w3.org/2001/03/webdata/xsv?docAddrs=http%3A%2F%2Fhome.arcor.de%2Fmartin.honnen%2Fxml%2FoneCharInstance1.xml+http%3A%2F%2Fhome.arcor.de%2Fmartin.honnen%2Fxml%2FoneCharSchema1.xsd&warnings=on&keepGoing=on&style=xsl#) don't report any validation error for the samples named above.