build.xml

<project name="Test" basedir="." default="build">
  <taskdef name="schematron" classname="name.dmaus.schxslt.ant.Task" classpath="target/ant-schxslt-1.9.5.jar"/>
  <target name="build">
    <schematron schema="schema.sch" file="document.xml" report="report.xml"/>
  </target>
</project>

document.xml

<document/>

schema.sch

<sch:schema xmlns:sch="http://purl.oclc.org/dsdl/schematron" queryBinding="xslt3">
  <sch:pattern>
    <sch:rule context="*">
      <sch:assert test="false()"/>
    </sch:rule>
  </sch:pattern>
</sch:schema>

All right. You need Saxon on your CLASSPATH. Could you add the Saxon HE jar to CLASSPATH and try again?

Sure!
(You mean to the bash environment variable $CLASSPATH, or to the ant @classpath on the <taskdef> for SchXslt?)

(BTW, I do not remember what I did yesterday to cause it, but I am now getting a different error:)

WARNING: A terminally deprecated method in java.lang.System has been called
WARNING: System::setSecurityManager has been called by org.apache.tools.ant.types.Permissions (file:/usr/share/ant/lib/ant.jar)
WARNING: Please consider reporting this to the maintainers of org.apache.tools.ant.types.Permissions
WARNING: System::setSecurityManager will be removed in a future release

Ah! Fixed that last one. Martin had warned me you sometimes need to specify fork="true" when calling Saxon via in ant. Sigh.